language: Refactor available_languages into its own struct (#61388)

This adds a dedicated `AvailableLanguages` struct in preparation for a
more cabable language matching based on a given language config. No
functional changes, just shuffling some code around for this round.

Also made the LanguageMatcher non-cloneable in favor of wrapping it in
an Arc, since cloning is rather expensive for this and having a
reference is sufficient in all cases right now.

Release Notes:

- N/A

---------

Co-authored-by: Ben Brandt <benjamin.j.brandt@gmail.com>

crates/git_ui/src/commit_view.rs

@@ -967,9 +967,9 @@ async fn build_buffer(
     let text = Rope::from(text);
     let language =
         cx.update(|_, cx| language_registry.language_for_file(&blob, Some(&text), cx))?;
-    let language = if let Some(language) = language {
+    let language = if let Some(language_id) = language {
         language_registry
-            .load_language(&language)
+            .load_language(language_id)
             .await
             .ok()
             .and_then(|e| e.log_err())