feat: add `cohere` backend

[maxbrunet]

Hello! 👋

This adds support for Cohere. API Reference: docs.cohere.com/reference/chat

I have not done a lot of experimentation yet, but it requires a custom preamble to work well for code completion since the default is design to be an assistant, it's a little too chatty. So far this is what I tested with:

You are a software engineer. Complete the code. Do not repeat the given code. Do not explain.

If this PR is accepted, I will document it and other default parameters in the editor extensions.

Disclaimer: I am a Cohere employee experimenting with the product, this is in no way indicates that Cohere will provide official support for llm-ls. 🙂