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/gpui/examples/window.rs

@@ -157,7 +157,7 @@ impl Render for WindowDemo {
 
                 // Restore the application after 3 seconds
                 window
-                    .spawn(cx, |mut cx| async move {
+                    .spawn(cx, async move |cx| {
                         Timer::after(std::time::Duration::from_secs(3)).await;
                         cx.update(|_, cx| {
                             cx.activate(false);