Design a custom feature / carver backend

[PersonTheCat]

This API should hook into the FEATURE chunk status--logic handled by ChunkGenerator#applyBiomeDecorations--to run a new kind of generator with the following features:

• Direct access to the ProtoChunk
• An optimized block placer which supports update-free block placement
• Outside-of-chunk biome predicates
• Any other features originally supported by Cave Generator

[PersonTheCat]

I'm pretty sure the way we're gonna handle this is by creating a custom type of net.minecraft.world.level.levelgen.feature.Feature which provides some additional data, most of which is actually stored directly in the SeverLevel, such as our NoiseGraph. Hypothetically, we can also use this API to run carvers. TBD.

If that's the path we take, the one issue we might run into is that heightmaps are calculated between carvers and features. we would need to pick a feature step and defer computing heightmaps to that point.