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/context_picker/thread_context_picker.rs

@@ -149,9 +149,9 @@ impl PickerDelegate for ThreadContextPickerDelegate {
             }
         });
 
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let matches = search_task.await;
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 this.delegate.matches = matches;
                 this.delegate.selected_index = 0;
                 cx.notify();
@@ -171,9 +171,9 @@ impl PickerDelegate for ThreadContextPickerDelegate {
 
         let open_thread_task = thread_store.update(cx, |this, cx| this.open_thread(&entry.id, cx));
 
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let thread = open_thread_task.await?;
-            this.update_in(&mut cx, |this, window, cx| {
+            this.update_in(cx, |this, window, cx| {
                 this.delegate
                     .context_store
                     .update(cx, |context_store, cx| context_store.add_thread(thread, cx))