(Bedrock Parity) Bring Data Driven Spawning to Java data packs

This is a feature I wanted to see for a long time - the ability to customize how and where mobs spawn in the world.

This was announced at Minecon 2018 for bedrock edition, but it could really help spice up java too, even without the custom entities from bedrock.

Besides the rules described on the wiki, I would like to see a rules for height coordinates. It would be useful to make the underground more dangerous, while keeping surface relatively safe.