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/go_to_line/src/cursor_position.rs

@@ -72,11 +72,9 @@ impl CursorPosition {
         cx: &mut Context<Self>,
     ) {
         let editor = editor.downgrade();
-        self.update_position = cx.spawn_in(window, |cursor_position, mut cx| async move {
+        self.update_position = cx.spawn_in(window, async move |cursor_position, cx| {
             let is_singleton = editor
-                .update(&mut cx, |editor, cx| {
-                    editor.buffer().read(cx).is_singleton()
-                })
+                .update(cx, |editor, cx| editor.buffer().read(cx).is_singleton())
                 .ok()
                 .unwrap_or(true);
 
@@ -87,7 +85,7 @@ impl CursorPosition {
             }
 
             editor
-                .update(&mut cx, |editor, cx| {
+                .update(cx, |editor, cx| {
                     cursor_position.update(cx, |cursor_position, cx| {
                         cursor_position.selected_count = SelectionStats::default();
                         cursor_position.selected_count.selections = editor.selections.count();