Refactor to use SharedString in more places (#23813)

Splitting this off from
https://github.com/zed-industries/zed/pull/23808, per @maxdeviant's
suggestion!

Release Notes:

- N/A

---------

Co-authored-by: Nathan <nathan@zed.dev>

crates/language/src/buffer_tests.rs

@@ -2424,7 +2424,7 @@ fn test_language_at_with_hidden_languages(cx: &mut App) {
             assert_eq!(config.language_name(), "Markdown".into());
 
             let language = snapshot.language_at(point).unwrap();
-            assert_eq!(language.name().0.as_ref(), "Markdown");
+            assert_eq!(language.name().as_ref(), "Markdown");
         }
 
         buffer