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/assistant2/src/context_picker/fetch_context_picker.rs

@@ -206,12 +206,12 @@ impl PickerDelegate for FetchContextPickerDelegate {
         let http_client = workspace.read(cx).client().http_client().clone();
         let url = self.url.clone();
         let confirm_behavior = self.confirm_behavior;
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let text = cx
                 .background_spawn(Self::build_message(http_client, url.clone()))
                 .await?;
 
-            this.update_in(&mut cx, |this, window, cx| {
+            this.update_in(cx, |this, window, cx| {
                 this.delegate
                     .context_store
                     .update(cx, |context_store, _cx| {

crates/assistant2/src/context_picker/file_context_picker.rs

@@ -206,11 +206,11 @@ impl PickerDelegate for FileContextPickerDelegate {
 
         let search_task = self.search(query, Arc::<AtomicBool>::default(), &workspace, cx);
 
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             // TODO: This should be probably be run in the background.
             let paths = search_task.await;
 
-            this.update(&mut cx, |this, _cx| {
+            this.update(cx, |this, _cx| {
                 this.delegate.matches = paths;
             })
             .log_err();
@@ -345,10 +345,10 @@ impl PickerDelegate for FileContextPickerDelegate {
         };
 
         let confirm_behavior = self.confirm_behavior;
-        cx.spawn_in(window, |this, mut cx| async move {
-            match task.await.notify_async_err(&mut cx) {
+        cx.spawn_in(window, async move |this, cx| {
+            match task.await.notify_async_err(cx) {
                 None => anyhow::Ok(()),
-                Some(()) => this.update_in(&mut cx, |this, window, cx| match confirm_behavior {
+                Some(()) => this.update_in(cx, |this, window, cx| match confirm_behavior {
                     ConfirmBehavior::KeepOpen => {}
                     ConfirmBehavior::Close => this.delegate.dismissed(window, cx),
                 }),

crates/assistant2/src/context_picker/thread_context_picker.rs

@@ -149,9 +149,9 @@ impl PickerDelegate for ThreadContextPickerDelegate {
             }
         });
 
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let matches = search_task.await;
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 this.delegate.matches = matches;
                 this.delegate.selected_index = 0;
                 cx.notify();
@@ -171,9 +171,9 @@ impl PickerDelegate for ThreadContextPickerDelegate {
 
         let open_thread_task = thread_store.update(cx, |this, cx| this.open_thread(&entry.id, cx));
 
-        cx.spawn_in(window, |this, mut cx| async move {
+        cx.spawn_in(window, async move |this, cx| {
             let thread = open_thread_task.await?;
-            this.update_in(&mut cx, |this, window, cx| {
+            this.update_in(cx, |this, window, cx| {
                 this.delegate
                     .context_store
                     .update(cx, |context_store, cx| context_store.add_thread(thread, cx))