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/project_symbols/src/project_symbols.rs

@@ -118,9 +118,9 @@ impl PickerDelegate for ProjectSymbolsDelegate {
             });
             let symbol = symbol.clone();
             let workspace = self.workspace.clone();
-            cx.spawn_in(window, |_, mut cx| async move {
+            cx.spawn_in(window, async move |_, cx| {
                 let buffer = buffer.await?;
-                workspace.update_in(&mut cx, |workspace, window, cx| {
+                workspace.update_in(cx, |workspace, window, cx| {
                     let position = buffer
                         .read(cx)
                         .clip_point_utf16(symbol.range.start, Bias::Left);
@@ -176,10 +176,10 @@ impl PickerDelegate for ProjectSymbolsDelegate {
         let symbols = self
             .project
             .update(cx, |project, cx| project.symbols(&query, cx));
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let symbols = symbols.await.log_err();
             if let Some(symbols) = symbols {
-                this.update_in(&mut cx, |this, window, cx| {
+                this.update_in(cx, |this, window, cx| {
                     let delegate = &mut this.delegate;
                     let project = delegate.project.read(cx);
                     let (visible_match_candidates, external_match_candidates) = symbols