Event: Add end-date

From our friend [Justin](https://wptavern.com/diving-into-automattics-block-experiments):

> One missing component I would like to see with this block is the ability to add both a start and end date. For multi-day events, users must provide that information in the description box, which would be acceptable for most use cases. However, the full event date would be better served via the “when” field.

Seems worth exploring!