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/editor/src/code_context_menus.rs

@@ -416,9 +416,9 @@ impl CompletionsMenu {
             cx,
         );
 
-        cx.spawn(move |editor, mut cx| async move {
+        cx.spawn(async move |editor, cx| {
             if let Some(true) = resolve_task.await.log_err() {
-                editor.update(&mut cx, |_, cx| cx.notify()).ok();
+                editor.update(cx, |_, cx| cx.notify()).ok();
             }
         })
         .detach();