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/project/src/debounced_delay.rs

@@ -35,7 +35,7 @@ impl<E: 'static> DebouncedDelay<E> {
         self.cancel_channel = Some(sender);
 
         let previous_task = self.task.take();
-        self.task = Some(cx.spawn(move |entity, mut cx| async move {
+        self.task = Some(cx.spawn(async move |entity, cx| {
             let mut timer = cx.background_executor().timer(delay).fuse();
             if let Some(previous_task) = previous_task {
                 previous_task.await;
@@ -46,7 +46,7 @@ impl<E: 'static> DebouncedDelay<E> {
                 _ = timer => {}
             }
 
-            if let Ok(task) = entity.update(&mut cx, |project, cx| (func)(project, cx)) {
+            if let Ok(task) = entity.update(cx, |project, cx| (func)(project, cx)) {
                 task.await;
             }
         }));