editor: Improve code completions by prioritizing prefix matching (#29456)

- Use common prefix length-based matching as primary criteria.
- Test added for multiple cases.

Before:
<img width="500" alt="image"
src="https://github.com/user-attachments/assets/8c653225-cac2-41bd-95f0-0fb8724284c9"
/>

After:
<img width="500" alt="image"
src="https://github.com/user-attachments/assets/a3d59399-cff2-435d-9b56-69a530f35da4"
/>

Release Notes:

- Fixed issues with code completions where they wouldn't show
completions with matched prefix at top.

crates/editor/src/editor_tests.rs

@@ -10732,7 +10732,7 @@ async fn test_completion(cx: &mut TestAppContext) {
             .confirm_completion(&ConfirmCompletion::default(), window, cx)
             .unwrap()
     });
-    cx.assert_editor_state("editor.clobberˇ");
+    cx.assert_editor_state("editor.closeˇ");
     handle_resolve_completion_request(&mut cx, None).await;
     apply_additional_edits.await.unwrap();
 }
@@ -13982,7 +13982,7 @@ async fn test_completions_in_languages_with_extra_word_characters(cx: &mut TestA
         {
             assert_eq!(
                 completion_menu_entries(&menu),
-                &["bg-blue", "bg-red", "bg-yellow"]
+                &["bg-red", "bg-blue", "bg-yellow"]
             );
         } else {
             panic!("expected completion menu to be open");