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/image_viewer/src/image_viewer.rs

@@ -179,15 +179,15 @@ impl Item for ImageView {
         _workspace_id: Option<WorkspaceId>,
         _: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<Entity<Self>>
+    ) -> Task<Option<Entity<Self>>>
     where
         Self: Sized,
     {
-        Some(cx.new(|cx| Self {
+        Task::ready(Some(cx.new(|cx| Self {
             image_item: self.image_item.clone(),
             project: self.project.clone(),
             focus_handle: cx.focus_handle(),
-        }))
+        })))
     }
 
     fn has_deleted_file(&self, cx: &App) -> bool {