Different Double Slabs

This release is compatible with all versions 1.20.0 and up!*

*The only caveat is that the resource pack won't support any new slabs added after 1.21, so message me on Discord reminding me to add any if I haven't released a new version yet! @owehttamy

Changes:

• Add culling to double slab variants
• Fixed red sandstone slab rendering incorrectly
• Reformatted and added files to help users create support for modded slabs