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/editor/src/signature_help.rs

@@ -179,10 +179,10 @@ impl Editor {
         let language = self.language_at(position, cx);
 
         self.signature_help_state
-            .set_task(cx.spawn_in(window, move |editor, mut cx| async move {
+            .set_task(cx.spawn_in(window, async move |editor, cx| {
                 let signature_help = task.await;
                 editor
-                    .update(&mut cx, |editor, cx| {
+                    .update(cx, |editor, cx| {
                         let Some(mut signature_help) = signature_help.into_iter().next() else {
                             editor
                                 .signature_help_state