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/slash_command_picker.rs

@@ -100,7 +100,7 @@ impl PickerDelegate for SlashCommandDelegate {
         cx: &mut Context<Picker<Self>>,
     ) -> Task<()> {
         let all_commands = self.all_commands.clone();
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let filtered_commands = cx
                 .background_spawn(async move {
                     if query.is_empty() {
@@ -119,7 +119,7 @@ impl PickerDelegate for SlashCommandDelegate {
                 })
                 .await;
 
-            this.update_in(&mut cx, |this, window, cx| {
+            this.update_in(cx, |this, window, cx| {
                 this.delegate.filtered_commands = filtered_commands;
                 this.delegate.set_selected_index(0, window, cx);
                 cx.notify();