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/collab_ui/src/chat_panel/message_editor.rs

@@ -137,11 +137,9 @@ impl MessageEditor {
         .detach();
 
         let markdown = language_registry.language_for_name("Markdown");
-        cx.spawn_in(window, |_, mut cx| async move {
+        cx.spawn_in(window, async move |_, cx| {
             let markdown = markdown.await.context("failed to load Markdown language")?;
-            buffer.update(&mut cx, |buffer, cx| {
-                buffer.set_language(Some(markdown), cx)
-            })
+            buffer.update(cx, |buffer, cx| buffer.set_language(Some(markdown), cx))
         })
         .detach_and_log_err(cx);
 
@@ -232,7 +230,7 @@ impl MessageEditor {
     ) {
         if let language::BufferEvent::Reparsed | language::BufferEvent::Edited = event {
             let buffer = buffer.read(cx).snapshot();
-            self.mentions_task = Some(cx.spawn_in(window, |this, cx| async move {
+            self.mentions_task = Some(cx.spawn_in(window, async move |this, cx| {
                 cx.background_executor()
                     .timer(MENTIONS_DEBOUNCE_INTERVAL)
                     .await;
@@ -251,7 +249,7 @@ impl MessageEditor {
             self.collect_mention_candidates(buffer, end_anchor, cx)
         {
             if !candidates.is_empty() {
-                return cx.spawn(|_, cx| async move {
+                return cx.spawn(async move |_, cx| {
                     Ok(Some(
                         Self::resolve_completions_for_candidates(
                             &cx,
@@ -270,7 +268,7 @@ impl MessageEditor {
             self.collect_emoji_candidates(buffer, end_anchor, cx)
         {
             if !candidates.is_empty() {
-                return cx.spawn(|_, cx| async move {
+                return cx.spawn(async move |_, cx| {
                     Ok(Some(
                         Self::resolve_completions_for_candidates(
                             &cx,
@@ -453,7 +451,7 @@ impl MessageEditor {
     async fn find_mentions(
         this: WeakEntity<MessageEditor>,
         buffer: BufferSnapshot,
-        mut cx: AsyncWindowContext,
+        cx: &mut AsyncWindowContext,
     ) {
         let (buffer, ranges) = cx
             .background_spawn(async move {
@@ -462,7 +460,7 @@ impl MessageEditor {
             })
             .await;
 
-        this.update(&mut cx, |this, cx| {
+        this.update(cx, |this, cx| {
             let mut anchor_ranges = Vec::new();
             let mut mentioned_user_ids = Vec::new();
             let mut text = String::new();