Add language_models crate to house language model providers (#20945)

This PR adds a new `language_models` crate to house the various language
model providers.

By extracting the provider definitions out of `language_model`, we're
able to remove `language_model`'s dependency on `editor`, which improves
incremental compilation when changing `editor`.

Release Notes:

- N/A

crates/language_model/src/provider.rs

@@ -1,8 +0,0 @@
-pub mod anthropic;
-pub mod cloud;
-pub mod copilot_chat;
-#[cfg(any(test, feature = "test-support"))]
-pub mod fake;
-pub mod google;
-pub mod ollama;
-pub mod open_ai;