Worldgen API v0 [Part 2]

[paulevsGitch]

This is the second worldgen PR. That one contains feature system. It also fixes some mistakes in original PR.

Basically all features are vanilla structure wrappers with simple hierarchy (used for some vanilla features), like that:

public static final Structure LAVA_LAKE_SCATTERED = new BottomWeightedScatter(
    new WeightedStructure(LAVA_LAKE, 8), 1, 8, 128
);

There are several scatter structures that will place inner structure, weighted structure that will deny placement based on weight and so on. Scatters support different techniques and can be used for Overworld, Nether, caves, custom dimensions and so on.

This PR is ready for review

[paulevsGitch]

Most default structures are located in DefaultStructures class, so making a biome that contains vanilla features should be easy

[mineLdiver]

Looks good. One comment though

[mineLdiver]

I approve. If this is ready to get merged, let's merge.