New excerpt controls (#24428)

Release Notes:

- Multibuffers now use less vertical space for excerpt boundaries.
Additionally the expand up/down arrows are hidden at the start and end
of the buffers

---------

Co-authored-by: Nate Butler <iamnbutler@gmail.com>
Co-authored-by: Zed AI <claude-3.5-sonnet@zed.dev>

crates/vim/src/test.rs

@@ -1728,14 +1728,7 @@ async fn test_folded_multibuffer_excerpts(cx: &mut gpui::TestAppContext) {
             ],
             cx,
         );
-        let mut editor = Editor::new(
-            EditorMode::Full,
-            multi_buffer.clone(),
-            None,
-            true,
-            window,
-            cx,
-        );
+        let mut editor = Editor::new(EditorMode::Full, multi_buffer.clone(), None, window, cx);
 
         let buffer_ids = multi_buffer.read(cx).excerpt_buffer_ids();
         // fold all but the second buffer, so that we test navigating between two