Add WorldBorder implementation

This PR implements the `WorldBorder` interface, as well as the `getWorldBorder` method of `World`

**EDIT**: The branch was not supposed to be called `scoreboard` - I'm just brain-dead.
This needs to be implemented below using an interface prefix as it has the same signature as our API. This would be fine in dev but the moment the source gets reobfuscated, this method will be come a searge name and no longer implement our interface.