Offhand: to place vertical slab/stairs without breaking old behaviors.

You can place a vertical slab/stair with your offhand.

Since we are more interested in its half faces rather than its complete faces, the logic of placing a vertical slab is not like those of existing rotatable incomplete blocks. We should orient the half face, instead of the complete face, to the player, so we can create delicate patterns easily. The base part of a stair is handled like a slab while its extra part always faces the player.