Added callback to harness 6D facedir in node definitions.

This allows a custom facedir handler to be used by user-defined nodes. For example, this could be used to allow upside down stairs without a dedicated node for them, as shown in the minimal game.

This could also be applied to upside down slabs, but figuring out how to do that without disrupting the current "slab + slab == full node" mechanic would be a challenge, and is beyond the scope of this pull request.