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/markdown_preview/src/markdown_preview_view.rs

@@ -372,13 +372,13 @@ impl MarkdownPreviewView {
     ) -> Task<Result<()>> {
         let language_registry = self.language_registry.clone();
 
-        cx.spawn_in(window, move |view, mut cx| async move {
+        cx.spawn_in(window, async move |view, cx| {
             if wait_for_debounce {
                 // Wait for the user to stop typing
                 cx.background_executor().timer(REPARSE_DEBOUNCE).await;
             }
 
-            let (contents, file_location) = view.update(&mut cx, |_, cx| {
+            let (contents, file_location) = view.update(cx, |_, cx| {
                 let editor = editor.read(cx);
                 let contents = editor.buffer().read(cx).snapshot(cx).text();
                 let file_location = MarkdownPreviewView::get_folder_for_active_editor(editor, cx);
@@ -389,7 +389,7 @@ impl MarkdownPreviewView {
                 parse_markdown(&contents, file_location, Some(language_registry)).await
             });
             let contents = parsing_task.await;
-            view.update(&mut cx, move |view, cx| {
+            view.update(cx, move |view, cx| {
                 let markdown_blocks_count = contents.children.len();
                 view.contents = Some(contents);
                 let scroll_top = view.list_state.logical_scroll_top();