Remove all windows before quitting

This gives all entities a chance of running `Drop::drop` which,
in turn, could cause them to spawn a critical task. For example,
we use this capability when a language server is dropped and we
need to asynchronously send a shutdown message.

crates/zed/src/main.rs

@@ -30,6 +30,7 @@ fn main() {
     languages.set_theme(&settings.borrow().theme.editor.syntax);
 
     app.on_quit(|cx| {
+        cx.remove_all_windows();
         let did_finish = cx
             .background()
             .block_on_critical_tasks(Duration::from_millis(100));