Add World Border API

This PR adds a `WorldBorder` interface for controlling per-world border.

There was an (unresolved) discussion on IRC as to whether or not to support per-**player** world borders.
I thought about that, but that would require putting the `z` position as the `y` coordinate of the vector. I thought that having a note in the javadoc is less confusing.

It always future-proofs the code, if Mojang ever allows world border to extend *part* of the y axis.