Use LanguageServerName in more places (#18167)

This pushes the new LanguageServerName type to more places.

As both languages and language servers were identified by Arc<str>, it
was
sometimes hard to tell which was intended.

Release Notes:

- N/A

crates/collab/src/tests/remote_editing_collaboration_tests.rs

@@ -102,7 +102,7 @@ async fn test_sharing_an_ssh_remote_project(
             all_language_settings(file, cx)
                 .language(Some(&("Rust".into())))
                 .language_servers,
-            ["override-rust-analyzer".into()]
+            ["override-rust-analyzer".to_string()]
         )
     });