[VS Code] Users Can Extend Language List for Code Suggestions

Problem to Solve

Based on the team's feedback, an MVC approach is to allow the user to extend the language list while we maintain the currently existing allowlists for Code Suggestions. By keeping our current implementation but adding an iteration, we can make a small change and see if users want to extend the current list of languages. We will measure if this is a direction we want to pursue by monitoring the number of unsupported languages we see for Code Suggestions in our current Tableau dashboards.

Proposal

1. Allow users to add their own language identifiers to their settings.json file in VS Code.

   As an example, here is an addition of gitlab.aiAssistedCodeSuggestions.languages to the settings.json file.

    "gitlab.debug": true,
    "gitlab.aiAssistedCodeSuggestions.languages": "markdown, scss, sql",
    "gitlab.customQueries": [
        
        {
            "name": "Issues assigned to me",
            "type": "issues",
            "scope": "assigned_to_me",
            "state": "opened",
            "noItemText": "No issues assigned to you."
        },

2. The default for each user is an empty list
3. The user can add language to the list by its identifier
4. For this MVC, additional in-IDE notifications are not necessary, but we'll need to update our documentation and direct users on how to add unsupported languages.