Add automation around contributing Copilot fig spec (terminal completions) to VS Code

[Tyriar]

Describe the feature or problem you'd like to solve

No response

Proposed solution

I'm adding a basic completion spec for copilot CLI in microsoft/vscode#272921, this will probably quickly go out of date and could give much richer completions. For example copilot --model could list the actual models using a generator.

Example prompts or workflows

No response

Additional context

No response