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/snippet_provider/src/lib.rs

@@ -181,8 +181,8 @@ impl SnippetProvider {
     fn watch_directory(&mut self, path: &Path, cx: &Context<Self>) {
         let path: Arc<Path> = Arc::from(path);
 
-        self.watch_tasks.push(cx.spawn(|this, mut cx| async move {
-            let fs = this.update(&mut cx, |this, _| this.fs.clone())?;
+        self.watch_tasks.push(cx.spawn(async move |this, cx| {
+            let fs = this.update(cx, |this, _| this.fs.clone())?;
             let watched_path = path.clone();
             let watcher = fs.watch(&watched_path, Duration::from_secs(1));
             initial_scan(this.clone(), path, cx.clone()).await?;