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/gpui/src/elements/div.rs

@@ -2485,7 +2485,7 @@ fn handle_tooltip_mouse_move(
                 let active_tooltip = active_tooltip.clone();
                 let build_tooltip = build_tooltip.clone();
                 let check_is_hovered_during_prepaint = check_is_hovered_during_prepaint.clone();
-                move |mut cx| async move {
+                async move |cx| {
                     cx.background_executor().timer(TOOLTIP_SHOW_DELAY).await;
                     cx.update(|window, cx| {
                         let new_tooltip =
@@ -2576,7 +2576,7 @@ fn handle_tooltip_check_visible_and_update(
         Action::ScheduleHide(tooltip) => {
             let delayed_hide_task = window.spawn(cx, {
                 let active_tooltip = active_tooltip.clone();
-                move |mut cx| async move {
+                async move |cx| {
                     cx.background_executor()
                         .timer(HOVERABLE_TOOLTIP_HIDE_DELAY)
                         .await;

crates/gpui/src/elements/img.rs

@@ -357,7 +357,7 @@ impl Element for Img {
                                     }
                                 } else {
                                     let current_view = window.current_view();
-                                    let task = window.spawn(cx, |mut cx| async move {
+                                    let task = window.spawn(cx, async move |cx| {
                                         cx.background_executor().timer(LOADING_DELAY).await;
                                         cx.update(move |_, cx| {
                                             cx.notify(current_view);