Custom Background Music

Custom music can now be set using datapacks
 - definitions are located in the "custom_bgm" folder, under the desired mod ID
 - a basic music provider is provided that takes a condition and list of tracks and selects one if the condition is true
 - provided conditions are:
 -- basic boolean logic (always, never, and, or, nand, nor, not)
 -- biome (can use tags and/or ids)
 -- combat (can specify tags and/or ids as well as the minimum and maximum number of entities targeting the player)
 - datagen helpers to come later
Update to FECore 1.3.0