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/remote_server/src/headless_project.rs

@@ -406,8 +406,8 @@ impl HeadlessProject {
         // and immediately dropping the reference of the new client, causing it
         // to be dropped on the headless project, and the client only then
         // receiving a response to AddWorktree.
-        cx.spawn(|mut cx| async move {
-            this.update(&mut cx, |this, cx| {
+        cx.spawn(async move |cx| {
+            this.update(cx, |this, cx| {
                 this.worktree_store.update(cx, |worktree_store, cx| {
                     worktree_store.add(&worktree, cx);
                 });
@@ -636,7 +636,7 @@ impl HeadlessProject {
         _envelope: TypedEnvelope<proto::ShutdownRemoteServer>,
         cx: AsyncApp,
     ) -> Result<proto::Ack> {
-        cx.spawn(|cx| async move {
+        cx.spawn(async move |cx| {
             cx.update(|cx| {
                 // TODO: This is a hack, because in a headless project, shutdown isn't executed
                 // when calling quit, but it should be.

crates/remote_server/src/unix.rs

@@ -272,7 +272,7 @@ fn start_server(
     })
     .detach();
 
-    cx.spawn(|cx| async move {
+    cx.spawn(async move |cx| {
         let mut stdin_incoming = listeners.stdin.incoming();
         let mut stdout_incoming = listeners.stdout.incoming();
         let mut stderr_incoming = listeners.stderr.incoming();
@@ -827,7 +827,7 @@ pub fn handle_settings_file_changes(
             .set_server_settings(&server_settings_content, cx)
             .log_err();
     });
-    cx.spawn(move |cx| async move {
+    cx.spawn(async move |cx| {
         while let Some(server_settings_content) = server_settings_file.next().await {
             let result = cx.update_global(|store: &mut SettingsStore, cx| {
                 let result = store.set_server_settings(&server_settings_content, cx);