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/session/src/session.rs

@@ -67,16 +67,14 @@ impl AppSession {
     pub fn new(session: Session, cx: &Context<Self>) -> Self {
         let _subscriptions = vec![cx.on_app_quit(Self::app_will_quit)];
 
-        let _serialization_task = Some(cx.spawn(|_, cx| async move {
-            loop {
-                if let Some(windows) = cx.update(|cx| cx.window_stack()).ok().flatten() {
-                    store_window_stack(windows).await;
-                }
-
-                cx.background_executor()
-                    .timer(Duration::from_millis(100))
-                    .await;
+        let _serialization_task = Some(cx.spawn(async move |_, cx| loop {
+            if let Some(windows) = cx.update(|cx| cx.window_stack()).ok().flatten() {
+                store_window_stack(windows).await;
             }
+
+            cx.background_executor()
+                .timer(Duration::from_millis(100))
+                .await;
         }));
 
         Self {