Entity spawning tables

We have loot tables for customizing item drops, why not have something similar for entity spawns?

Entity tables should syntax similar to loot tables, but with the item field turned to an entity field. A few tweaks would probably be needed, but not a lot of major stuff.

Under this change, there would be spawning tables for all biomes, which control what entities can spawn there. In addition, you could use them for mob spawners (making the syntax for making custom ones MUCH easier to work with), the mobs spawned when infested blocks are broken, and what spawns when a slime is killed. This would let data packs have a greater ability to customize how the game plays.