Feature: Object placement in caves

[Jishuna]

A new placement option for objects allowing them to target the top or bottom of caves, rather than relying on randomY and block checks.

The new vanilla resources in 1.17 and 1.18 may have some code to help us implement this.

[PG85]

Could think about making a new placement setting like highestblock, that instead uses

• StartY
• NearestSurface: Above/Below
• MinY / MaxY

That could be used in any situation where you can predict the y of caves. If startY is air, finds the nearest surface above/below (like highestblock).

[FrankTCA]

Also would be nice for my nether preset since that's a thing now. Could use this feature for sure.