As the tin says. Also stops a error with SDate on new games.

As we discussed on Discord, we don't have a use case yet for an AfterNewGameIntroStarted event. I'll prevent AfterDayStarted from being raised on spring 0 (and prevent Context.IsWorldReady from being set), and we can discuss adding an event or context flag later if needed.

Done in develop for the SMAPI 2.0 release.