Support Codeium indexing #544

Support a proof-of-concept version of GitHub Copilot chat #550. Please note that this feature is still in an early stage of development and may change in the future. To use it, you have to turn on "Pretend IDE to be VSCode" in the GitHub Copilot settings and change the chat model to "GitHub Copilot (poc)".

This implementation uses the language server to access the GitHub Copilot chat, so it should be much more legal than copilot-gpt4-service. The only(?) not-so-legal thing is that we need to use VSCode as the IDE name. Custom system prompt and temperature will not work for GitHub Copilot chat. Scopes will work. The built-in RAG of GitHub Copilot chat may not work properly.

Bump Copilot.vim to 1.37.0 and fixes #530

Add a "Utility chat model" picker in the chat settings. This model will be used when it need to use LLM as a tool.

Prevent repeated parsing of markdown content in chat panel on scroll.

Move appcast.xml to https://copilotforxcode.intii.com/appcast.xml