Feature request: True purple shulker box variants

[IchHabeHunger54]

This is not a bug, I just didn't know where to put this.

1.13 re-added the purple shulker box alongside the undyed shulker box. However, purple iron shulker boxes do not exist (well, they do, but with the undyed shulker box texture). I figured that might be something to improve the mod.