SGB SOU_TRN demo

This tool can be used to test the different SNES preset instrument sounds that come with the SGB.
It plays a short custom music track.
Tested on PAL SNES.

Project source: https://github.com/cloudsmon/sgb_nspc_music

The source is probably more interesting than the tool itself...

My entry for #gbcompo23

@judges: This only works on SGB, don't bother running on CGB