Persist events that not yet have a handler

There's a risk that a event is emitted before its' handler has been registered. You might say that such code is bad design, but I'd like to be lazy ;) Also, given that code splitting is being more and more adopted, I think this scenario is very real.

Would it be possible to persist events that don't have a handler (yet)? Whenever `.on(...)` is run, mitt would then check the persisted events in order to find unprocessed ones, and then handle those events immediately.

I think the footprint for this change can be quite small, but I can't speak for performance. Maybe we can opt-in to the behaviour.

Lee el punto de entrada actual del emisor de TypeScript y sus pruebas existentes para entender cómo se comportan emit y .on(...) cuando no hay ningún handler registrado. Define el comportamiento de persistencia opt-in, incluidas las expectativas de orden, replay y rendimiento; se considera completado cuando los eventos no gestionados se conservan y se entregan cuando se registra posteriormente un handler, sin cambiar el comportamiento predeterminado.

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