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/diagnostics/src/buffer_diagnostics.rs

@@ -693,11 +693,11 @@ impl Item for BufferDiagnosticsEditor {
         _workspace_id: Option<workspace::WorkspaceId>,
         window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<Entity<Self>>
+    ) -> Task<Option<Entity<Self>>>
     where
         Self: Sized,
     {
-        Some(cx.new(|cx| {
+        Task::ready(Some(cx.new(|cx| {
             BufferDiagnosticsEditor::new(
                 self.project_path.clone(),
                 self.project.clone(),
@@ -706,7 +706,7 @@ impl Item for BufferDiagnosticsEditor {
                 window,
                 cx,
             )
-        }))
+        })))
     }
 
     fn deactivated(&mut self, window: &mut Window, cx: &mut Context<Self>) {