[#2056] Better signature items support

Motivation and Context

Improved support for handling interfaces and signatures (module types) in the language server.

Some improvements are still due for a future MR:

• Fix "go to type definition" to properly deal with signature items.
• Handle top-level and module includes.

Initial signature items support was added in !2946 (merged).

Related issues

Partially handles #2056 (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

Changes are described in detail for each commit.

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

Made various improvements to signatures and interfaces in the language server:

• Changed "go to definition" to try to find the signatures/interfaces that define some module element.
• Implemented "go to implementations" to try to find the modules/namespaces that implement some signature item.
  • Also added support to look for implementations from signature names themselves.
• Implemented "go to declaration" to point to where a variable was originally defined (mimics the original "go to definition" behavior before this patch).
• Fixed hovers to properly distinguish between modules/namespaces and signatures/interfaces.
• Fixed "find references" (and "rename") to include signature items.
• [VSCode/Vim/Emacs extensions] Fixed syntax highlighting for signatures and interfaces.

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).