[#1698][#1674] Make LSP hovers prettier

Motivation and Context

As in #1698 (closed), LSP hovers can be too big, or can even contain invalid code.

As in #1674 (closed) they could be prettier.

Related issues

Resolves #1698 (closed). Resolves #1674 (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

• Use e.g. x : int or type t = int instead of just int for hovers
• Change the untyper so it could use var names to construct morec ompact types e.g. a -> b instead of (A of int) -> (B of bool) when type a = A of int and type b = B of bool
• Send hovers in MarkedString, as described in modern LSP protocol docs
• Improve the decompiler so it can create the Ast_unified.type_expression from ASt_core.type_expression even when the first expression contain quantifiers
• Improve the decompiler for CameLIGO so the hover could be int -> int -> int instead of int -> (int -> int)

TODO: two hovers simultaneosly in case of orig_var like x : bool and type x = True | False Looks impossible to implement before #1943 is resolved and looks like go-to-type-definition is enough to understgand meaning of a type variable.

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

Improve the LSP hovers

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).