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/diagnostics/src/items.rs

@@ -163,12 +163,12 @@ impl DiagnosticIndicator {
             .map(|entry| entry.diagnostic);
         if new_diagnostic != self.current_diagnostic {
             self.diagnostics_update =
-                cx.spawn_in(window, |diagnostics_indicator, mut cx| async move {
+                cx.spawn_in(window, async move |diagnostics_indicator, cx| {
                     cx.background_executor()
                         .timer(Duration::from_millis(50))
                         .await;
                     diagnostics_indicator
-                        .update(&mut cx, |diagnostics_indicator, cx| {
+                        .update(cx, |diagnostics_indicator, cx| {
                             diagnostics_indicator.current_diagnostic = new_diagnostic;
                             cx.notify();
                         })