When using typescript and listening for youtube statechange event, it is missing the code property definition inside the event (even though it is provided in the code).
By making events a map of key-value, we can add easily custom event type for specific event name. Since YouTube "statechange" event differs from the basic PlyrEvent, I added a new Event Type "PlyrStateChangeEvent" having a code property corresponding to a YoutubeState enum defined by the YouTube API documentation.
This pattern follows how addEventListener in the lib.dom.d.ts is defined.
