Add support for other chat providers, such as Tabby

[bendavis78]

Tabby is a Copilot alternative that supports the use of private LLMs It has API endpoints for completion and chat_completions . A vim plugin exists for completion, but not for in-editor chat.

It should be relatively simple to abstract the existing API calls to a pluggable completion "backend", and move the Copilot-specific calls into its own backend. A config setting could then be used to select the preferred completion backend.