Replaced several core classes from DD with new classes to enforce
integrity checks. Rewriting everything that depended on those classes is
a massive undertaking but it should simplify our code and prevent the
many bugs we've seen lately. The rewrite isn't done yet, just committing
my progress so far.
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.
Moved CommonTickHandler into the ticking package in preparation for the
next change. Renamed CommandStartDungeonCreation to CommandCreatePocket,
since creation pockets of variable sizes will be its future role now
that it's not strictly necessary for exporting. Renamed
CommandEndDungeonCreation to CommandExportDungeon, since we no longer
have a dungeon creation process like before.
