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/scroll.rs

@@ -341,12 +341,12 @@ impl ScrollManager {
         }
 
         if cx.default_global::<ScrollbarAutoHide>().0 {
-            self.hide_scrollbar_task = Some(cx.spawn_in(window, |editor, mut cx| async move {
+            self.hide_scrollbar_task = Some(cx.spawn_in(window, async move |editor, cx| {
                 cx.background_executor()
                     .timer(SCROLLBAR_SHOW_INTERVAL)
                     .await;
                 editor
-                    .update(&mut cx, |editor, cx| {
+                    .update(cx, |editor, cx| {
                         editor.scroll_manager.show_scrollbars = false;
                         cx.notify();
                     })
@@ -425,9 +425,9 @@ impl Editor {
         let opened_first_time = self.scroll_manager.visible_line_count.is_none();
         self.scroll_manager.visible_line_count = Some(lines);
         if opened_first_time {
-            cx.spawn_in(window, |editor, mut cx| async move {
+            cx.spawn_in(window, async move |editor, cx| {
                 editor
-                    .update(&mut cx, |editor, cx| {
+                    .update(cx, |editor, cx| {
                         editor.refresh_inlay_hints(InlayHintRefreshReason::NewLinesShown, cx)
                     })
                     .ok()