Created NewLinkData and replaced references to the original LinkData.
Moved it to the mod_pocketDim.core package. Added Point4D, an immutable
point type for 3D integer coordinates with an added dimension ID.
Made progress on overhauling and prettifying our commands. There are
still more changes to be done for this to be functional. I need to do an
intermediate commit because I want to merge in recent changes by Steven.
