workspace: Make Item::clone_on_split async (#41211)

Split out from https://github.com/zed-industries/zed/pull/40774 to
reduce the size of the reland of that PR (once I figure out the cause of
the issue)

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/editor/src/items.rs

@@ -762,11 +762,11 @@ impl Item for Editor {
         _workspace_id: Option<WorkspaceId>,
         window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<Entity<Editor>>
+    ) -> Task<Option<Entity<Editor>>>
     where
         Self: Sized,
     {
-        Some(cx.new(|cx| self.clone(window, cx)))
+        Task::ready(Some(cx.new(|cx| self.clone(window, cx))))
     }
 
     fn set_nav_history(