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/collab_ui/src/channel_view.rs

@@ -498,8 +498,8 @@ impl Item for ChannelView {
         _: Option<WorkspaceId>,
         window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<Entity<Self>> {
-        Some(cx.new(|cx| {
+    ) -> Task<Option<Entity<Self>>> {
+        Task::ready(Some(cx.new(|cx| {
             Self::new(
                 self.project.clone(),
                 self.workspace.clone(),
@@ -508,7 +508,7 @@ impl Item for ChannelView {
                 window,
                 cx,
             )
-        }))
+        })))
     }
 
     fn navigate(