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/buffer_diff/src/buffer_diff.rs

@@ -1080,12 +1080,12 @@ impl BufferDiff {
         let complete_on_drop = util::defer(|| {
             tx.send(()).ok();
         });
-        cx.spawn(|_, mut cx| async move {
+        cx.spawn(async move |_, cx| {
             let snapshot = snapshot.await;
             let Some(this) = this.upgrade() else {
                 return;
             };
-            this.update(&mut cx, |this, _| {
+            this.update(cx, |this, _| {
                 this.set_state(snapshot, &buffer);
             })
             .log_err();