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/Cargo.toml

@@ -61,6 +61,7 @@ install_cli.workspace = true
 journal.workspace = true
 language.workspace = true
 language_model.workspace = true
+language_models.workspace = true
 language_selector.workspace = true
 language_tools.workspace = true
 languages = { workspace = true, features = ["load-grammars"] }