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/zed/src/zed.rs

@@ -2854,14 +2854,16 @@ mod tests {
         });
 
         // Split the pane with the first entry, then open the second entry again.
-        window
+        let (task1, task2) = window
             .update(cx, |w, window, cx| {
-                w.split_and_clone(w.active_pane().clone(), SplitDirection::Right, window, cx);
-                w.open_path(file2.clone(), None, true, window, cx)
+                (
+                    w.split_and_clone(w.active_pane().clone(), SplitDirection::Right, window, cx),
+                    w.open_path(file2.clone(), None, true, window, cx),
+                )
             })
-            .unwrap()
-            .await
             .unwrap();
+        task1.await.unwrap();
+        task2.await.unwrap();
 
         window
             .read_with(cx, |w, cx| {
@@ -3484,7 +3486,13 @@ mod tests {
                     SplitDirection::Right,
                     window,
                     cx,
-                );
+                )
+            })
+            .unwrap()
+            .await
+            .unwrap();
+        window
+            .update(cx, |workspace, window, cx| {
                 workspace.open_path(
                     (worktree.read(cx).id(), rel_path("the-new-name.rs")),
                     None,

crates/zed/src/zed/component_preview.rs

@@ -720,7 +720,7 @@ impl Item for ComponentPreview {
         _workspace_id: Option<WorkspaceId>,
         window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<gpui::Entity<Self>>
+    ) -> Task<Option<gpui::Entity<Self>>>
     where
         Self: Sized,
     {
@@ -742,13 +742,13 @@ impl Item for ComponentPreview {
             cx,
         );
 
-        match self_result {
+        Task::ready(match self_result {
             Ok(preview) => Some(cx.new(|_cx| preview)),
             Err(e) => {
                 log::error!("Failed to clone component preview: {}", e);
                 None
             }
-        }
+        })
     }
 
     fn to_item_events(event: &Self::Event, mut f: impl FnMut(workspace::item::ItemEvent)) {