refactor: add state manager to LS suggestions + react to enabled

Description

This MR adds state manager to the LS integration. It also introduces a middleware that:

• prevents "standard" completion requests from being sent at all
• prevents inline completion requests from being sent if the code suggestions are disabled

What is left to implement after this MR:

• tracking loading and error states
• introduce a "no account" state

Related Issues

Resolves #1088 (closed)

Partially implements: #1052 (closed)

How has this been tested?

1. Disable code suggestions in your settings and make sure you use the LS implementation

   {
     "gitlab.aiAssistedCodeSuggestions.enabled": false,
     "gitlab.featureFlags.languageServer":true,
   }

2. See that suggestions are disabled in the status bar

   • 

3. Click the status bar icon

   • see that the gitlab.aiAssistedCodeSuggestions.enabled value changed in your settings.json to true
   • see suggestion enabled in you status bar
     • 

4. Click the status bar icon again

   • see code suggestions were disabled only for this session
   • 
   • go to JS/TS file and see that no suggestions are coming in

5. Click again to enable

6. Go to a JSON file

   • see that the language is unsupported
   • 

7. Go to JS file and type something in

   • see suggestions coming in

Screenshots (if appropriate):

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 change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap