Support substitution variables in prompt files

[burkeholland]

Describe the feature you'd like

Allow custom prompt files in VS Code to use substitution variables, so that arguments can be passed by position when invoking a prompt.

Example usage:

/review-pr 456 high alice

And in the prompt source:

Review PR #$1 with priority $2 and assign to $3

Source inspiration: u/El-Paul on Reddit

Somewhat Related: #271125

[vs-code-engineering]

Hi @burkeholland. As a member of the team, you can help us triage this issue by referring to https://github.com/microsoft/vscode-copilot/wiki/Copilot-Inbox-Tracker and assigning an owner directly.

[aeschli]

The LLM us already smart enough in doing that without any substitution mechanisms from the outside:

Also note the use of argument-hint that gives a nice UI:

[aeschli]

Ok to close?