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/semantic_index/examples/index.rs

@@ -55,18 +55,18 @@ fn main() {
             api_key,
         ));
 
-        cx.spawn(|mut cx| async move {
+        cx.spawn(async move |cx| {
             let semantic_index = SemanticDb::new(
                 PathBuf::from("/tmp/semantic-index-db.mdb"),
                 embedding_provider,
-                &mut cx,
+                cx,
             );
 
             let mut semantic_index = semantic_index.await.unwrap();
 
             let project_path = Path::new(&args[1]);
 
-            let project = Project::example([project_path], &mut cx).await;
+            let project = Project::example([project_path], cx).await;
 
             cx.update(|cx| {
                 let language_registry = project.read(cx).languages().clone();
@@ -113,7 +113,7 @@ fn main() {
                         let worktree = search_result.worktree.read(cx);
                         let entry_abs_path = worktree.abs_path().join(search_result.path.clone());
                         let fs = project.read(cx).fs().clone();
-                        cx.spawn(|_| async move { fs.load(&entry_abs_path).await.unwrap() })
+                        cx.spawn(async move |_| fs.load(&entry_abs_path).await.unwrap())
                     })
                     .unwrap()
                     .await;