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/livekit_client_macos/examples/test_app_macos.rs

@@ -37,7 +37,7 @@ fn main() {
         let live_kit_key = std::env::var("LIVE_KIT_KEY").unwrap_or("devkey".into());
         let live_kit_secret = std::env::var("LIVE_KIT_SECRET").unwrap_or("secret".into());
 
-        cx.spawn(|cx| async move {
+        cx.spawn(async move |cx| {
             let user_a_token = token::create(
                 &live_kit_key,
                 &live_kit_secret,