pub fn subscribe(&self) -> Option<SignalSubscriber> {
        self.state
            .lock()
            .expect("poisoned lock")
            .add_subscriber()
            .map(SignalSubscriber)
    }
Register a new subscriber
Will return `None` if SignalHandler is in the process of shutting down
or if it has already shut down.
crates/turborepo-signals/src/lib.rs