[0.17.2] Allow Selectively Disabling/Enabling Terrain Types

[MiniHerc]

In 0.16.x, we could create an all desert or all grassland world by setting the other terrain types to <none> in the world generation.

In 0.17.x, we can only make the other terrain types less likely to generate.

I preferred being able to disable terrain types that I didn't want to see in a world. Could we please get some way to do so? And yes, I want to deliberately create a 100% grassland / forest world in 0.17.x. With lakes and oceans, of course.