File protocol handler for installing custom instructions, prompts, and chat modes

[aaronpowell]

With VS Code, they support redirecting to vscode:/chat-prompt/install?url=<path to prompt file>, with that pattern repeated across the other styles of customisation.

This has enabled us to have a very smooth integration point with the https://github.com/github/awesome-copilot repo to enable people to add customisations to their repos.

It'd be great to have a comparable feature for the Xcode plugin, to enable people to easily share and acquire copilot customisations.