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/auto_update/src/auto_update.rs

@@ -252,11 +252,9 @@ impl AutoUpdater {
     }
 
     pub fn start_polling(&self, cx: &mut Context<Self>) -> Task<Result<()>> {
-        cx.spawn(|this, mut cx| async move {
-            loop {
-                this.update(&mut cx, |this, cx| this.poll(cx))?;
-                cx.background_executor().timer(POLL_INTERVAL).await;
-            }
+        cx.spawn(async move |this, cx| loop {
+            this.update(cx, |this, cx| this.poll(cx))?;
+            cx.background_executor().timer(POLL_INTERVAL).await;
         })
     }
 
@@ -267,9 +265,9 @@ impl AutoUpdater {
 
         cx.notify();
 
-        self.pending_poll = Some(cx.spawn(|this, mut cx| async move {
+        self.pending_poll = Some(cx.spawn(async move |this, cx| {
             let result = Self::update(this.upgrade()?, cx.clone()).await;
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 this.pending_poll = None;
                 if let Err(error) = result {
                     log::error!("auto-update failed: error:{:?}", error);