[#2021] Allow user to choose faster implementations for LSP completions

Motivation and Context

As in #2021 (closed), Ligo_interface.get_scopes is slow, so our LSP can stuck when its asked for completions and opened file is big enough.

Related issues

Resolves #2021 (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:
  • If I implemented a new LSP request, I added it to the list of supported features that may be disabled
  • If I implemented a new LSP method, I added it to the list of supported functionality
• 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

Add two new implementations for completions that don't require such heavy computations. Add an option so user can choose one of three behaviors.

Also updates description of our VSCode extension since we're making a new release anyway

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

Added new faster implementations for the LSP completion request. If LIGO LSP stuck while computing completions, you can choose other implementations in the VSCode extension settings.

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).