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/src/markdown.rs

@@ -231,10 +231,10 @@ impl Markdown {
         });
 
         self.should_reparse = false;
-        self.pending_parse = Some(cx.spawn(|this, mut cx| {
+        self.pending_parse = Some(cx.spawn(async move |this, cx| {
             async move {
                 let parsed = parsed.await?;
-                this.update(&mut cx, |this, cx| {
+                this.update(cx, |this, cx| {
                     this.parsed_markdown = parsed;
                     this.pending_parse.take();
                     if this.should_reparse {
@@ -246,6 +246,7 @@ impl Markdown {
                 anyhow::Ok(())
             }
             .log_err()
+            .await
         }));
     }
 
@@ -795,7 +796,7 @@ impl Element for MarkdownElement {
                                                     code.clone(),
                                                 ));
 
-                                                cx.spawn(|this, cx| async move {
+                                                cx.spawn(async move |this, cx| {
                                                     cx.background_executor()
                                                         .timer(Duration::from_secs(2))
                                                         .await;