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/workspace/src/item.rs

@@ -718,13 +718,13 @@ impl<T: Item> ItemHandle for Entity<T> {
 
                 send_follower_updates = Some(cx.spawn_in(window, {
                     let pending_update = pending_update.clone();
-                    |workspace, mut cx| async move {
+                    async move |workspace, cx| {
                         while let Some(mut leader_id) = pending_update_rx.next().await {
                             while let Ok(Some(id)) = pending_update_rx.try_next() {
                                 leader_id = id;
                             }
 
-                            workspace.update_in(&mut cx, |workspace, window, cx| {
+                            workspace.update_in(cx, |workspace, window, cx| {
                                 let Some(item) = item.upgrade() else { return };
                                 workspace.update_followers(
                                     is_project_item,