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/assistant/Cargo.toml

@@ -50,6 +50,7 @@ indexed_docs.workspace = true
 indoc.workspace = true
 language.workspace = true
 language_model.workspace = true
+language_models.workspace = true
 log.workspace = true
 lsp.workspace = true
 markdown.workspace = true