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/language_tools/src/syntax_tree_view.rs

@@ -3,7 +3,7 @@ use editor::{Anchor, Editor, ExcerptId, SelectionEffects, scroll::Autoscroll};
 use gpui::{
     App, AppContext as _, Context, Div, Entity, EntityId, EventEmitter, FocusHandle, Focusable,
     Hsla, InteractiveElement, IntoElement, MouseButton, MouseDownEvent, MouseMoveEvent,
-    ParentElement, Render, ScrollStrategy, SharedString, Styled, UniformListScrollHandle,
+    ParentElement, Render, ScrollStrategy, SharedString, Styled, Task, UniformListScrollHandle,
     WeakEntity, Window, actions, div, rems, uniform_list,
 };
 use language::{Buffer, OwnedSyntaxLayer};
@@ -573,17 +573,17 @@ impl Item for SyntaxTreeView {
         _: 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| {
             let mut clone = Self::new(self.workspace_handle.clone(), None, window, cx);
             if let Some(editor) = &self.editor {
                 clone.set_editor(editor.editor.clone(), window, cx)
             }
             clone
-        }))
+        })))
     }
 }