The string-replace function was added in Emacs 28 and we want to
retain compatibility with older versions.

Fix documentation lookup

See merge request !33

add fennel-scratch command

See merge request !32

This command creates a new *fennel-scratch* buffer for quick Fennel
evaluation.  It works much like inbuilt *scratch* buffer with the
lisp-interaction-mode.

Inherit from prog-mode instead of lisp-mode

As was noted in #11 `lisp-mode` is for *common* lisp-compatible languages, which Fennel is not part of.

This MR changes the inheritance to `prog-mode`, but we still rely on a lot of things from `lisp-mode`, but we have to manually set those up.

This change should not break anything, but hooks defined for `lisp-mode-hook` will stop working and should be instead set via `fennel-mode-hook`

See merge request !31

indent tables by data, not by (curly)braces

See merge request !30

fennel-repl: reuse repl buffer if repl process is dead, persist fennel-program locally

See merge request !29

fennel-find-definition: use (symbol-at-point) as the default value

See merge request !28