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/channel/src/channel_buffer.rs

@@ -47,7 +47,7 @@ impl ChannelBuffer {
         client: Arc<Client>,
         user_store: Entity<UserStore>,
         channel_store: Entity<ChannelStore>,
-        mut cx: AsyncApp,
+        cx: &mut AsyncApp,
     ) -> Result<Entity<Self>> {
         let response = client
             .request(proto::JoinChannelBuffer {
@@ -66,7 +66,7 @@ impl ChannelBuffer {
             let capability = channel_store.read(cx).channel_capability(channel.id);
             language::Buffer::remote(buffer_id, response.replica_id as u16, capability, base_text)
         })?;
-        buffer.update(&mut cx, |buffer, cx| buffer.apply_ops(operations, cx))?;
+        buffer.update(cx, |buffer, cx| buffer.apply_ops(operations, cx))?;
 
         let subscription = client.subscribe_to_entity(channel.id.0)?;
 
@@ -208,7 +208,7 @@ impl ChannelBuffer {
         let client = self.client.clone();
         let epoch = self.epoch();
 
-        self.acknowledge_task = Some(cx.spawn(move |_, cx| async move {
+        self.acknowledge_task = Some(cx.spawn(async move |_, cx| {
             cx.background_executor()
                 .timer(ACKNOWLEDGE_DEBOUNCE_INTERVAL)
                 .await;