More tweaks to JsLIGO

Motivation and Context

The recent refactoring of JsLIGO was a bit rushed towards the end. Some small ironing is needed.

Description

• [JsLIGO] Documented deep pattern matching.
• [CameLIGO] Documented loops + enabled downto.
• [CameLIGO] Added let mut to sequences in the parser.
• [PascaLIGO] Removal of code snippets in the documentation.
• Small fixes.

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

• [JsLIGO] Documented do { ... } expression (in pattern matching).
• [JsLIGO] Fixed pretty-printing of constant constructors.
• [JsLIGO] Fixed spacing after pretty-printing of contract_of.

Checklist:

• If a new syntax has been introduced, put a message on slack ligo-lsp
• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).