[#2120] Implement autocompletion for CLI

Motivation and Context

Problem

We want to have autocompletions for the ligo CLI. Looks like Core.Command supports it out of the box, so, let's polish CLI code a bit.

Solution

Strip the Command_unx module and replace it with core_unix.command_unix. Motivation is simple: Obj.magic inside Command_unx results in segfault. Add some argument types with static completions.

Related issues

Resolves #2120 (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

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 CLI autocompletion script. All the instructions are available in README.md

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).