pub fn new_async(
		action_handler: impl (Fn(ActionHandler) -> Box<dyn Future<Output = ActionHandler> + Send + Sync>)
			+ Send
			+ Sync
			+ 'static,
	) -> Result<Arc<Self>, CriticalError> {
		let config = Config::default();
		config.on_action_async(action_handler);
		Self::with_config(config).map(Arc::new)
	}
Instantiates a new `Watchexec` runtime given an initial async action handler.

This is the same as [`new`](fn@Self::new) except the action handler is async.
crates/lib/src/watchexec.rs