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/assistant_eval/src/eval.rs

@@ -63,14 +63,14 @@ impl Eval {
         model: Arc<dyn LanguageModel>,
         cx: &mut App,
     ) -> Task<anyhow::Result<EvalOutput>> {
-        cx.spawn(move |mut cx| async move {
+        cx.spawn(async move |cx| {
             checkout_repo(&self.eval_setup, &self.repo_path).await?;
 
             let (assistant, done_rx) =
                 cx.update(|cx| HeadlessAssistant::new(app_state.clone(), cx))??;
 
             let _worktree = assistant
-                .update(&mut cx, |assistant, cx| {
+                .update(cx, |assistant, cx| {
                     assistant.project.update(cx, |project, cx| {
                         project.create_worktree(&self.repo_path, true, cx)
                     })
@@ -79,7 +79,7 @@ impl Eval {
 
             let start_time = std::time::SystemTime::now();
 
-            assistant.update(&mut cx, |assistant, cx| {
+            assistant.update(cx, |assistant, cx| {
                 assistant.thread.update(cx, |thread, cx| {
                     let context = vec![];
                     thread.insert_user_message(self.user_prompt.clone(), context, cx);
@@ -93,7 +93,7 @@ impl Eval {
 
             let diff = query_git(&self.repo_path, vec!["diff"]).await?;
 
-            assistant.update(&mut cx, |assistant, cx| {
+            assistant.update(cx, |assistant, cx| {
                 let thread = assistant.thread.read(cx);
                 let last_message = thread.messages().last().unwrap();
                 if last_message.role != language_model::Role::Assistant {