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/workspace/src/theme_preview.rs

@@ -1,5 +1,7 @@
 #![allow(unused, dead_code)]
-use gpui::{AnyElement, App, Entity, EventEmitter, FocusHandle, Focusable, Hsla, actions, hsla};
+use gpui::{
+    AnyElement, App, Entity, EventEmitter, FocusHandle, Focusable, Hsla, Task, actions, hsla,
+};
 use strum::IntoEnumIterator;
 use theme::all_theme_colors;
 use ui::{
@@ -100,11 +102,11 @@ impl Item for ThemePreview {
         _workspace_id: Option<crate::WorkspaceId>,
         window: &mut Window,
         cx: &mut Context<Self>,
-    ) -> Option<Entity<Self>>
+    ) -> Task<Option<Entity<Self>>>
     where
         Self: Sized,
     {
-        Some(cx.new(|cx| Self::new(window, cx)))
+        Task::ready(Some(cx.new(|cx| Self::new(window, cx))))
     }
 }