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/src/tests/test_server.rs

@@ -208,8 +208,8 @@ impl TestServer {
             .unwrap()
             .set_id(user_id.to_proto())
             .override_authenticate(move |cx| {
-                cx.spawn(|_| async move {
-                    let access_token = "the-token".to_string();
+                let access_token = "the-token".to_string();
+                cx.spawn(async move |_| {
                     Ok(Credentials {
                         user_id: user_id.to_proto(),
                         access_token,
@@ -230,7 +230,7 @@ impl TestServer {
                 let connection_killers = connection_killers.clone();
                 let forbid_connections = forbid_connections.clone();
                 let client_name = client_name.clone();
-                cx.spawn(move |cx| async move {
+                cx.spawn(async move |cx| {
                     if forbid_connections.load(SeqCst) {
                         Err(EstablishConnectionError::other(anyhow!(
                             "server is forbidding connections"