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/editor/src/editor_tests.rs

@@ -9312,6 +9312,7 @@ async fn test_word_completion(cx: &mut TestAppContext) {
             .server
             .on_request::<lsp::request::Completion, _, _>(move |_, cx| {
                 let lsp_throttle_completions = lsp_throttle_completions.clone();
+                let cx = cx.clone();
                 async move {
                     if lsp_throttle_completions.load(atomic::Ordering::Acquire) {
                         cx.background_executor()