Connectivity for two dimensions along a given plane

[haykam821]

The BlockTraversal utilities are primarily meant for three dimensions, but often only two dimensions need to be traversed, such as to flood-fill the floor of a map. 4-connectivity and 8-connectivity could be supported, with variants for each three-dimensional axis. Alternatively, only the XZ-plane needs to be supported.

Note that the BlockTraversal.Connectivity.create method is not public, so custom connectivities cannot be used.