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/multi_buffer/src/multi_buffer.rs

@@ -1752,7 +1752,7 @@ impl MultiBuffer {
             .detach()
         }
 
-        cx.spawn(move |this, mut cx| async move {
+        cx.spawn(async move |this, cx| {
             let mut results_by_buffer_id = HashMap::default();
             while let Some((buffer_id, buffer, ranges, excerpt_ranges, range_counts)) =
                 excerpt_ranges_rx.next().await
@@ -1772,7 +1772,7 @@ impl MultiBuffer {
                 let mut ranges = ranges.into_iter();
                 let mut range_counts = range_counts.into_iter();
                 for excerpt_ranges in excerpt_ranges.chunks(100) {
-                    let excerpt_ids = match this.update(&mut cx, |this, cx| {
+                    let excerpt_ids = match this.update(cx, |this, cx| {
                         this.push_excerpts(buffer.clone(), excerpt_ranges.iter().cloned(), cx)
                     }) {
                         Ok(excerpt_ids) => excerpt_ids,