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/editor/src/items.rs

@@ -129,13 +129,8 @@ impl FollowableItem for Editor {
                 });
 
                 cx.new(|cx| {
-                    let mut editor = Editor::for_multibuffer(
-                        multibuffer,
-                        Some(project.clone()),
-                        true,
-                        window,
-                        cx,
-                    );
+                    let mut editor =
+                        Editor::for_multibuffer(multibuffer, Some(project.clone()), window, cx);
                     editor.remote_id = Some(remote_id);
                     editor
                 })