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/languages/src/python.rs

@@ -339,10 +339,10 @@ impl ContextProvider for PythonContextProvider {
         };
 
         let worktree_id = location.buffer.read(cx).file().map(|f| f.worktree_id(cx));
-        cx.spawn(move |mut cx| async move {
+        cx.spawn(async move |cx| {
             let active_toolchain = if let Some(worktree_id) = worktree_id {
                 toolchains
-                    .active_toolchain(worktree_id, "Python".into(), &mut cx)
+                    .active_toolchain(worktree_id, "Python".into(), cx)
                     .await
                     .map_or_else(
                         || "python3".to_owned(),