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/zed/src/main.rs

@@ -387,7 +387,8 @@ fn main() {
             cx,
         );
         supermaven::init(app_state.client.clone(), cx);
-        language_model::init(
+        language_model::init(cx);
+        language_models::init(
             app_state.user_store.clone(),
             app_state.client.clone(),
             app_state.fs.clone(),