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/assistant_context_editor/src/context_history.rs

@@ -164,9 +164,9 @@ impl PickerDelegate for SavedContextPickerDelegate {
         cx: &mut Context<Picker<Self>>,
     ) -> Task<()> {
         let search = self.store.read(cx).search(query, cx);
-        cx.spawn(|this, mut cx| async move {
+        cx.spawn(async move |this, cx| {
             let matches = search.await;
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 let host_contexts = this.delegate.store.read(cx).host_contexts();
                 this.delegate.matches = host_contexts
                     .iter()