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

### 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completion spec for copilot CLI in https://github.com/microsoft/vscode/issues/272921, this will probably quickly go out of date and could give much richer completions.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_