levels.json format

[thatsmydoing]

This is a bit of an open ticket. We carry a lot of baggage from typingmania's format. In particular, the names of fields don't really make too much sense.

There were also some changes I made that don't make sense in hindsight, particularly having start/end fields for the lines when in practice just having a single "start time" would be better. Even the editor exposes it that way.