Add a split button to terminal panes (#21251)

Follow-up of https://github.com/zed-industries/zed/pull/21238

<img width="873" alt="image"
src="https://github.com/user-attachments/assets/8cf2d8ea-a1df-4a6a-95d6-5867e0ee287d">

Release Notes:

- N/A

crates/workspace/src/pane.rs

@@ -303,7 +303,7 @@ pub struct Pane {
     double_click_dispatch_action: Box<dyn Action>,
     save_modals_spawned: HashSet<EntityId>,
     pub new_item_context_menu_handle: PopoverMenuHandle<ContextMenu>,
-    split_item_context_menu_handle: PopoverMenuHandle<ContextMenu>,
+    pub split_item_context_menu_handle: PopoverMenuHandle<ContextMenu>,
     pinned_tab_count: usize,
 }