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/livekit_client/src/remote_video_track_view.rs

@@ -29,15 +29,15 @@ impl RemoteVideoTrackView {
         Self {
             track,
             latest_frame: None,
-            _maintain_frame: cx.spawn_in(window, |this, mut cx| async move {
+            _maintain_frame: cx.spawn_in(window, async move |this, cx| {
                 futures::pin_mut!(frames);
                 while let Some(frame) = frames.next().await {
-                    this.update(&mut cx, |this, cx| {
+                    this.update(cx, |this, cx| {
                         this.latest_frame = Some(frame);
                         cx.notify();
                     })?;
                 }
-                this.update(&mut cx, |_this, cx| {
+                this.update(cx, |_this, cx| {
                     #[cfg(not(target_os = "macos"))]
                     {
                         use util::ResultExt as _;