terminal completion provider extension API

[Tyriar]

Now that the terminal suggest is getting more reliable and feature complete, we should look into what a completion provider could look like in the extension API. Some examples of where this would be useful:

• Providing completions for shells we don't support yet, like integrating fig completion specs.
• A provider for git that added smarts beyond what we can do in pwsh. For example it could complete git clone https:// to git clone https://github.com/ and provide the user's orgs/forks when completing https://github.com/, etc.

[Tyriar]

@meganrogge you might be tracking this one elsewhere

[meganrogge]

I used this #226562 to track the proposal, will keep this one to track for finalization

[meganrogge]

API sync feedback:

• look into why cwd is optional but somehow mandatory
• look into why env is on resource request config
• pass in id to registerCompletionProvider vs having id on the provider
• ensure it's clear that id needs to be unique, mention where it will show up (setting, hover w cmd +/ )
• add comments and examples jsdoc
• consider glob pattern vs file extensions?

[Tyriar]

API sync feedback (mostly the same as above):

• Add docs for everything, explain commandLine, cursorPosition, replacement* etc.
• TerminalCompletionProvider.id
  • id should instead me an arg in in call to register the provider
  • Explain requirements: unique to extension, globally?
  • Do we need to prefix the ID with the extension ID internally so extensions won't step on each other?
  • Hint where id shows up for the user (providers setting, ctrl+/ debug info)
• TerminalResourceRequestConfig.env
  • Wasn't clear to me what this does?
• TerminalResourceRequestConfig.cwd
  • Is it mandatory or not? Comment contradicts somewhat
• Buffer access likely won't change commandLine
• TerminalResourceRequestConfig.fileExtensions
  • Should we use RelativePattern?