Extension API for contribute Custom Chat Modes

[Skn0tt]

I work on playwright-vscode, and I'd like to be able to contribute a "Testing" mode to Copilot from that Extension. It can be used to generate new tests and debug existing ones. It contains a testing-specific base prompt and recommends the playwright-mcp tools (#251459 would also be useful for that). Ideally I'd be able to contribute an entire MCP instance that's only enabled in this mode, as opposed to a list of tools.

In that mode, I wouldn't want the tool list to be the definitive list though - i'd still want the agent to have access to user-configured tools. So it should serve more as a bias than as a definitive list.