Make Rich Soil use a HashMap for Mushroom Colonies

[Ultrasquid9]

This allows addons to add their own Mushroom Colonies without needing to mixin RichSoilBlock.

This also changes the MushroomColonyBlock to use an Item instead of a Holder<Item>, as the builtInRegistryHolder method is deprecated and using an Item works fine.

[thelegitdolt]

i suport this