Peek flat shulker icons

This adds support for Peek 1.2.0, while maintaining support for previous
versions (1.1.12)

Since Minecraft uses custom_model_data in game, spawning an item with
`/give @p minecraft:birch_log{"peek:shulker_item":1}` to match Peek’s
rendering wont show the same item as our resource pack, so we still
maintain `/give @p minecraft:birch_log{CustomModelData:1702}`