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/snippets_ui/src/snippets_ui.rs

@@ -134,13 +134,13 @@ impl PickerDelegate for ScopeSelectorDelegate {
             let language = self.language_registry.language_for_name(&scope_name);
 
             if let Some(workspace) = self.workspace.upgrade() {
-                cx.spawn_in(window, |_, mut cx| async move {
+                cx.spawn_in(window, async move |_, cx| {
                     let scope = match scope_name.as_str() {
                         "Global" => "snippets".to_string(),
                         _ => language.await?.lsp_id(),
                     };
 
-                    workspace.update_in(&mut cx, |workspace, window, cx| {
+                    workspace.update_in(cx, |workspace, window, cx| {
                         workspace
                             .open_abs_path(
                                 config_dir().join("snippets").join(scope + ".json"),
@@ -187,7 +187,7 @@ impl PickerDelegate for ScopeSelectorDelegate {
     ) -> gpui::Task<()> {
         let background = cx.background_executor().clone();
         let candidates = self.candidates.clone();
-        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()
@@ -211,7 +211,7 @@ impl PickerDelegate for ScopeSelectorDelegate {
                 .await
             };
 
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 let delegate = &mut this.delegate;
                 delegate.matches = matches;
                 delegate.selected_index = delegate