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/terminal_inline_assistant.rs

@@ -825,7 +825,7 @@ impl PromptEditor {
         let Some(model) = LanguageModelRegistry::read_global(cx).active_model() else {
             return;
         };
-        self.pending_token_count = cx.spawn(|this, mut cx| async move {
+        self.pending_token_count = cx.spawn(async move |this, cx| {
             cx.background_executor().timer(Duration::from_secs(1)).await;
             let request =
                 cx.update_global(|inline_assistant: &mut TerminalInlineAssistant, cx| {
@@ -833,7 +833,7 @@ impl PromptEditor {
                 })??;
 
             let token_count = cx.update(|cx| model.count_tokens(request, cx))?.await?;
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 this.token_count = Some(token_count);
                 cx.notify();
             })
@@ -1140,7 +1140,7 @@ impl Codegen {
         let telemetry = self.telemetry.clone();
         self.status = CodegenStatus::Pending;
         self.transaction = Some(TerminalTransaction::start(self.terminal.clone()));
-        self.generation = cx.spawn(|this, mut cx| async move {
+        self.generation = cx.spawn(async move |this, cx| {
             let model_telemetry_id = model.telemetry_id();
             let model_provider_id = model.provider_id();
             let response = model.stream_completion_text(prompt, &cx).await;
@@ -1197,12 +1197,12 @@ impl Codegen {
                     }
                 });
 
-                this.update(&mut cx, |this, _| {
+                this.update(cx, |this, _| {
                     this.message_id = message_id;
                 })?;
 
                 while let Some(hunk) = hunks_rx.next().await {
-                    this.update(&mut cx, |this, cx| {
+                    this.update(cx, |this, cx| {
                         if let Some(transaction) = &mut this.transaction {
                             transaction.push(hunk, cx);
                             cx.notify();
@@ -1216,7 +1216,7 @@ impl Codegen {
 
             let result = generate.await;
 
-            this.update(&mut cx, |this, cx| {
+            this.update(cx, |this, cx| {
                 if let Err(error) = result {
                     this.status = CodegenStatus::Error(error);
                 } else {