New feat.: localize new note's front matter / theme selection

* Localize new note's front matter

* Theme selection:
  A theme is an interchangeable set of configuration file variables. Tp-Note is shipped
  with two scheme: `default`, `zettel` (for Zettelkasten, experimental).

  The user can select a scheme with:
  * the command line option `--scheme SCHEME_NAME`,
  * environment variable `TPNOTE_SCHEME` or
  * the configuration file variable `arg_default.scheme`.

* The command line option:
  `--force-lang='-'` is replaced with `--force-lang=''`

Internal improvements: better modelling of "sequential/chronological" sort-tags

This release formalizes the concepts of `sequential sort-tags` vs.
`chronological sort-tags`:

1. A _sort-tag_ is composed of a number of counters, which can be numerical,
   e.g. `123.28` or combined numerical/letter based, e.g. `123ab`.

2. A counter is set of digits (base 10) `123` or a set of lowercase
   letters (base 26) `ab`.

3. A letter based counter can be maximal 2 letters wide. Its maximum is
   `zz` (cf. `filename.sort_tag.letters_in_succession_max`).

4. A _sequential sort-tag_ is a sort-tag that whose counters are at most 3
   digits wide (cf. `sort_tag.sequential.digits_in_succession_max`).

5. The filter `incr_sort_tag` increments only sequential sort-tags.

6. In order not to confuse sequential and chronological sort-tags, it is
   recommended to always write out the year in chronological sort-tags with 4
   digits, e.g. `2013-08-10` or `20130810`.

Tmpl: relax sort-tag chars; increment sequential sort-tags;

New features:

1. Default change in `filename.sort_tag_chars`: sort-tags may now include
   lowercase letters. The space and tab characters are depreciated.
2. Tp-Note searches for the latest created document, extracts the sort-tag from
   the finding, increments it and uses the result as sort-tag for the new file.
   Incrementation examples: `02-3` -> `02-4` or `1b01f` -> `1b01g`.
   Chronological sort-tags like `20231024` are never incremented.
3. The viewer now understands so-called _format strings_ that can be appended
   to local links, e.g. the link `<tpnote:1a3?-->` expands to
   `<a href="1a3-my doc.md">my doc</a>` in case a file `1a3-my doc.md` exists in
   the current directory.

Breaking changes:

1. In previous Tp-Note versions, the default set for allowed characters in
   sort-tags included the space and tab characters. From this version on, these
   characters are not allowed in valid sort-tags anymore. If needed, you can
   re-add them to the configuration file variable `filename.sort_tag_chars`.

Minor improvements in Markdown parsing

The version now recognizes all the official Markdown escape `\`
characters when searching for Hyperlinks or rendering `.txtnote` files.

See
[CommonMark Spec](https://spec.commonmark.org/0.30/#backslash-escapes)
for more details.

Viewer renderer: resolve shorthand links to their link text

* Viewer: When viewing documents, shortcut links are resolved to their title.
  For example: a shorthand link `tpnote:docs/20230513` that targets the file
  `docs/20230513-Animals.md` is shown as a hyperlink with the link text
  "Animals" in the browser.

* Viewer: hyperlinks with embedded images are now supported.

* The `.txtnote` renderer now parses inline images and hyperlinks with inline
  images.

Viewer: link to other docs with shorthand links, minor regr. fixes

New features:

* A shorthand link in a Tp-Note document e.g. `[text](tpnote:123)` is now
  expanded when being viewed. The renderer searches for a file starting
  with the sort-tag `123` on disk and displays the filename (without sort-tag).
  For example: a shorthand link to the file `123-My Animals.md` can be
  written as`<tpnote:123>` and the link will be displayed as `My Animals`.
* HTML rendition: The erroneous link path is now shown next to the error
  message.

(Minor) bug fixes:

* `file_ext: ''` is allowed again (regression introduced in v1.22.0)
* The viewer opens also (with an error message) in case of syntax errors
  in `file_ext:` or `sort_tag:`. These syntax errors had been mistakenly
  considered being fatal since v1.22.0.

Internal improvements:

* The link rewriting code is heavily refactored, simplified and easier to
  maintain.

New shortcut notation to link Tp-Note documents

* Links in Tp-Note documents: Add new optional scheme `tpnote:` for local links,
  e.g. in Markdown the autolink `<tpnote:dir/my%20other%20doc.md>` targets
  `dir/my other doc.md`. The link is displayed as `my other doc`.
* Configuration: the variable `filename.extensions` groups all former
  `filename.extensions_*` variables (config breaking change).
* Viewer: internal improvements handling HTML escape codes and percent codes
  in URLs.
* Integration: register additional Tp-Note files extension: `.txtnote`. The
  viewer only links, no markup.
* Template: new filter: `link_text_picky`, similar to `link_text`. The latter
  always return the fist link found, `link_text_picky` rejects obvious
  meaningless link text content.

Improve `.txtnote` renderer: recognize Markdown autolink

`.txtnote` renderer: Recognize Markdown autolinks, e.g. <https:getreu.net> or <foo@mydomain.com>

Breaking configuration change:

* Rename `tmpl_html.linux.*` variables to `tmpl_html.unix.*`

Add compilation target `freebsd`, fixes #17

Maintenance release: refactor and simplify HTML tmpl. code

Breaking change in HTML templates:

* Rename template variables `{{note_*}}` to `{{ doc_* }}`
* New `markup_to_html(extension=ext)` filter
* Replace template variable `doc_erroneous_content_html|safe` with
  `doc_text|markup_to_html|safe`

Viewer: choose syntax highlighting theme

This release come with 3 new configuration features:

1. Choose the syntax highlighting theme with the configuration file variables:
   `tmpl_html.viewer_highlighting_theme` and
   `tmpl_html.exporter_highlighting_theme`.

2. Breaking change: The HTML templates `tmpl_html.viewer`,
   `tmpl_html.viewer_error` and `tmpl_html.exporter` automatically escape
   HTML critical characters. Escaping can be disabled be adding the
   `safe` filter in last position.

3. Tp-Note's configuration file is now stored with inline documentation.

Maintenance release, no new features

Configuration breaking change:

* Split the `field(...)` filter into `remove(key=...)` and
  `insert(key=..., value=...)` filters with more common names.

Support also nested YAML types (map, array) in document header

Before this release, all header variables were limited to flat string types.
This release overcomes this limitation: all templates and the document viewer
can now deal with all YAML types, e.g. String, Number, Bool, including the
nested ones: Array, Object. For example, the following is now valid:

```yaml
title:     My Article
author:
  first:   John Doe
  second:  Martin Foo
date:
- 2023-04-13
- 2023-04-22
```

Further improvements:

* The templates only emits double quotes, were required be YAML standards.

* Configurable type checks occur before applying filename templates (see
  `tmpl.filter_assert_preconditions` in `tpnote-lib/src/config_default.toml`).

* Stronger (configurable) type checks for `title:`, `subtitle:`,
  `lang:`, `sort_tag:` and `file_ext:` header variables with user prompts

* The configuration defaults are now written and documented in TOML format:
  * `tpnote/src/config_default.toml`
  * `tpnote-lib/src/config_default.toml`

Maintenance release: move crate `tp-note` to `tpnote`. No code change.

Fix filename related edge cases

This release fixes some filename related edge cases:

* Prepend `filename.sort_tag_extra_separator` in case the note's title
  field is empty.
* Append `filename.copy_counter_extra_separator` in case the file stem
  resembles a copy counter.

Fix a minor regression when the title contains only sort-tag characters

This release fixes a minor regression introduced with Tp-Note v1.21.10: Note
files, created with title fields containing only sort tag characters, need an
`extra_separator` between in the sort-tag and the title in their filename.
Failing this rule caused cyclic filename change when synchronizing the note's
metadata and the filename. Now, an extra-separator is correctly inserted
as before.

Fix minor regression: ref docs by sort-tag only

A minor regression introduced with Tp-Note v1.21.11
prevented the viewer from following links to other Tp-Note
documents by indicating the sort-tag only. This commit
restores the previous behavior.

Maintenance release: remove helper fn from public API

Fix for minor regression introduced with v1.21.10

The incomplete refactoring of filename related code in v1.21.10 led to
some buggy behavior in some edge cases.

Improve the sort-tag detection algorithm

This release adds the optional configuration file variable
`filename.sort_tag_separator="-"` which specifies how the sort tag is separated
from the rest of the filename without being part of the former. As a result, the
additional `filename.sort_tag_extra_separator="''"` is required less often.