Let modders add custom noteblock instruments

This change lets modders add custom vanilla noteblock-compatible instruments using EnumHelper and stuff. So a modder could add a `GUITAR("pling")` instrument, then hook noteblock events, check the block below, and change it into `GUITAR("pling")`.

Existing noteblock mods should be compatible with this change, but without support for extra instruments (that is, custom instruments should play as `PIANO("harp")`).