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/assistant/src/assistant.rs

@@ -98,9 +98,9 @@ pub fn init(
     AssistantSettings::register(cx);
     SlashCommandSettings::register(cx);
 
-    cx.spawn(|mut cx| {
+    cx.spawn({
         let client = client.clone();
-        async move {
+        async move |cx| {
             let is_search_slash_command_enabled = cx
                 .update(|cx| cx.wait_for_flag::<SearchSlashCommandFeatureFlag>())?
                 .await;
@@ -116,7 +116,7 @@ pub fn init(
             let semantic_index = SemanticDb::new(
                 paths::embeddings_dir().join("semantic-index-db.0.mdb"),
                 Arc::new(embedding_provider),
-                &mut cx,
+                cx,
             )
             .await?;