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/editor/src/proposed_changes_editor.rs

@@ -77,7 +77,7 @@ impl ProposedChangesEditor {
             title: title.into(),
             buffer_entries: Vec::new(),
             recalculate_diffs_tx,
-            _recalculate_diffs_task: cx.spawn_in(window, |this, mut cx| async move {
+            _recalculate_diffs_task: cx.spawn_in(window, async move |this, cx| {
                 let mut buffers_to_diff = HashSet::default();
                 while let Some(mut recalculate_diff) = recalculate_diffs_rx.next().await {
                     buffers_to_diff.insert(recalculate_diff.buffer);
@@ -99,7 +99,7 @@ impl ProposedChangesEditor {
                     }
 
                     let recalculate_diff_futures = this
-                        .update(&mut cx, |this, cx| {
+                        .update(cx, |this, cx| {
                             buffers_to_diff
                                 .drain()
                                 .filter_map(|buffer| {