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/feedback/src/feedback.rs

@@ -48,7 +48,7 @@ pub fn init(cx: &mut App) {
             .register_action(|_, _: &CopySystemSpecsIntoClipboard, window, cx| {
                 let specs = SystemSpecs::new(window, cx);
 
-                cx.spawn_in(window, |_, mut cx| async move {
+                cx.spawn_in(window, async move |_, cx| {
                     let specs = specs.await.to_string();
 
                     cx.update(|_, cx| {
@@ -67,7 +67,7 @@ pub fn init(cx: &mut App) {
                 .detach();
             })
             .register_action(|_, _: &RequestFeature, window, cx| {
-                cx.spawn_in(window, |_, mut cx| async move {
+                cx.spawn_in(window, async move |_, cx| {
                     cx.update(|_, cx| {
                         cx.open_url(&request_feature_url());
                     })
@@ -77,7 +77,7 @@ pub fn init(cx: &mut App) {
             })
             .register_action(move |_, _: &FileBugReport, window, cx| {
                 let specs = SystemSpecs::new(window, cx);
-                cx.spawn_in(window, |_, mut cx| async move {
+                cx.spawn_in(window, async move |_, cx| {
                     let specs = specs.await;
                     cx.update(|_, cx| {
                         cx.open_url(&file_bug_report_url(&specs));