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/assistant2/src/message_editor.rs

@@ -206,10 +206,10 @@ impl MessageEditor {
 
         let thread = self.thread.clone();
         let context_store = self.context_store.clone();
-        cx.spawn(move |_, mut cx| async move {
+        cx.spawn(async move |_, cx| {
             refresh_task.await;
             thread
-                .update(&mut cx, |thread, cx| {
+                .update(cx, |thread, cx| {
                     let context = context_store.read(cx).snapshot(cx).collect::<Vec<_>>();
                     thread.insert_user_message(user_message, context, cx);
                     thread.send_to_model(model, request_kind, cx);
@@ -297,9 +297,9 @@ impl MessageEditor {
             .thread
             .update(cx, |thread, cx| thread.report_feedback(is_positive, cx));
 
-        cx.spawn(|_, mut cx| async move {
+        cx.spawn(async move |_, cx| {
             report.await?;
-            workspace.update(&mut cx, |workspace, cx| {
+            workspace.update(cx, |workspace, cx| {
                 let message = if is_positive {
                     "Positive feedback recorded. Thank you!"
                 } else {