Cleanup logic for registering languages and grammars (#7593)

This is a refactor, follow-up to the work we've been doing on loading
WASM language extensions.

Release Notes:

- N/A

---------

Co-authored-by: Marshall <marshall@zed.dev>

crates/language/src/buffer_tests.rs

@@ -5,6 +5,7 @@ use crate::language_settings::{
 use crate::Buffer;
 use clock::ReplicaId;
 use collections::BTreeMap;
+use futures::FutureExt as _;
 use gpui::{AppContext, Model};
 use gpui::{Context, TestAppContext};
 use indoc::indoc;