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/editor/src/clangd_ext.rs

@@ -51,7 +51,7 @@ pub fn switch_source_header(
             cx,
         )
     });
-    cx.spawn_in(window, |_editor, mut cx| async move {
+    cx.spawn_in(window, async move |_editor, cx| {
         let switch_source_header = switch_source_header_task
             .await
             .with_context(|| format!("Switch source/header LSP request for path \"{source_file}\" failed"))?;
@@ -72,7 +72,7 @@ pub fn switch_source_header(
         })?;
 
         workspace
-            .update_in(&mut cx, |workspace, window, cx| {
+            .update_in(cx, |workspace, window, cx| {
                 workspace.open_abs_path(path, OpenOptions { visible: Some(OpenVisible::None), ..Default::default() }, window, cx)
             })
             .with_context(|| {