Move "async move" a few characters to the left in cx.spawn() (#26758)

This is the core change:
https://github.com/zed-industries/zed/pull/26758/files#diff-044302c0d57147af17e68a0009fee3e8dcdfb4f32c27a915e70cfa80e987f765R1052

TODO:
- [x] Use AsyncFn instead of Fn() -> Future in GPUI spawn methods
- [x] Implement it in the whole app
- [x] Implement it in the debugger 
- [x] Glance at the RPC crate, and see if those box future methods can
be switched over. Answer: It can't directly, as you can't make an
AsyncFn* into a trait object. There's ways around that, but they're all
more complex than just keeping the code as is.
- [ ] Fix platform specific code

Release Notes:

- N/A

crates/workspace/src/searchable.rs

@@ -303,7 +303,7 @@ impl<T: SearchableItem> SearchableItemHandle for Entity<T> {
         cx: &mut App,
     ) -> Task<AnyVec<dyn Send>> {
         let matches = self.update(cx, |this, cx| this.find_matches(query, window, cx));
-        window.spawn(cx, |_| async {
+        window.spawn(cx, async |_| {
             let matches = matches.await;
             let mut any_matches = AnyVec::with_capacity::<T::Match>(matches.len());
             {