Explore 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