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/src/summary_index.rs

@@ -422,7 +422,7 @@ impl SummaryIndex {
     ) -> MightNeedSummaryFiles {
         let fs = self.fs.clone();
         let (rx, tx) = channel::bounded(2048);
-        let task = cx.spawn(|cx| async move {
+        let task = cx.spawn(async move |cx| {
             cx.background_executor()
                 .scoped(|cx| {
                     for _ in 0..cx.num_cpus() {
@@ -490,7 +490,7 @@ impl SummaryIndex {
         cx: &App,
     ) -> SummarizeFiles {
         let (summarized_tx, summarized_rx) = channel::bounded(512);
-        let task = cx.spawn(|cx| async move {
+        let task = cx.spawn(async move |cx| {
             while let Ok(file) = unsummarized_files.recv().await {
                 log::debug!("Summarizing {:?}", file);
                 let summary = cx
@@ -564,7 +564,7 @@ impl SummaryIndex {
         };
 
         let code_len = code.len();
-        cx.spawn(|cx| async move {
+        cx.spawn(async move |cx| {
             let stream = model.stream_completion(request, &cx);
             cx.background_spawn(async move {
                 let answer: String = stream