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/theme_selector/src/icon_theme_selector.rs

@@ -218,7 +218,7 @@ impl PickerDelegate for IconThemeSelectorDelegate {
             .map(|(id, meta)| StringMatchCandidate::new(id, &meta.name))
             .collect::<Vec<_>>();
 
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let matches = if query.is_empty() {
                 candidates
                     .into_iter()
@@ -242,7 +242,7 @@ impl PickerDelegate for IconThemeSelectorDelegate {
                 .await
             };
 
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 this.delegate.matches = matches;
                 this.delegate.selected_index = this
                     .delegate

crates/theme_selector/src/theme_selector.rs

@@ -266,7 +266,7 @@ impl PickerDelegate for ThemeSelectorDelegate {
             .map(|(id, meta)| StringMatchCandidate::new(id, &meta.name))
             .collect::<Vec<_>>();
 
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let matches = if query.is_empty() {
                 candidates
                     .into_iter()
@@ -290,7 +290,7 @@ impl PickerDelegate for ThemeSelectorDelegate {
                 .await
             };
 
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 this.delegate.matches = matches;
                 this.delegate.selected_index = this
                     .delegate