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/feature_flags/src/feature_flags.rs

@@ -252,7 +252,7 @@ impl FeatureFlagAppExt for App {
     fn wait_for_flag_or_timeout<T: FeatureFlag>(&mut self, timeout: Duration) -> Task<bool> {
         let wait_for_flag = self.wait_for_flag::<T>();
 
-        self.spawn(|_cx| async move {
+        self.spawn(async move |_cx| {
             let mut wait_for_flag = wait_for_flag.fuse();
             let mut timeout = FutureExt::fuse(smol::Timer::after(timeout));