Sticky multibuffer headers (#22391)

https://github.com/user-attachments/assets/92cc5ff7-d8be-4e4b-ac6e-68eb310fffce

Release Notes:

- Multibuffer headers will now stick to the top of the viewport as you
scroll
- Added support for expanding diagnostic excerpts

---------

Co-authored-by: Michael <michael@zed.dev>

crates/diagnostics/src/diagnostics.rs

@@ -166,7 +166,7 @@ impl ProjectDiagnosticsEditor {
         let excerpts = cx.new_model(|cx| MultiBuffer::new(project_handle.read(cx).capability()));
         let editor = cx.new_view(|cx| {
             let mut editor =
-                Editor::for_multibuffer(excerpts.clone(), Some(project_handle.clone()), false, cx);
+                Editor::for_multibuffer(excerpts.clone(), Some(project_handle.clone()), true, cx);
             editor.set_vertical_scroll_margin(5, cx);
             editor
         });