Add C++ configuration as a language model tool

[benmcmorran]

This is essentially the same as #12044, but adapted to use the new proposed language model tools API instead of the chat variables API. I'm leaving it in a draft statue until the API is on a path to stabilization and we've resolved issues like https://github.com/microsoft/vscode/issues/225737. I'm also guessing we'll want to add the ability to A/B experiment.

cc @lukka @sinemakinci1