[#1648] Implement folding range for the LSP

Motivation and Context

Implements "folding range".

Resolves #1648 (closed)

✅ Checklist for the LIGO Language Server

• I checked whether I need to update the README.md file for the plugin and did so if necessary
• I checked that my changes work in Emacs, Vim, and Visual Studio Code
• (Before merging) The commit history is squashed and prettified, and follows the Serokell commit policy, or the MR is set to squash the commits

Description

Implements "folding range" from LSP, which provides a list of ranges that allow a language client to collapse or expand regions of code.

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

Folding ranges allow the language server client to collapse or expand regions of code. Although some clients like Visual Studio Code can try to automatically deduce ranges based on regions, others like Emacs or Vim might not, so we implement the folding range requests from LSP in ligo lsp.

Checklist:

• 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).