Allow importing music in ABC format

There is an existing library called abclua but it requires the LPeg library which is a C module
https://code.google.com/archive/p/abclua/
This can't be used as-is in WoW

ABC format is convenient since it's already plain text and widely used in other games such as Lord of the Rings Online. Unfortunately, it's implementation is very complex and not worth the time. BTW it's still possible to convert ABC into MIDI then import it in Musician.