Bdubs Mecha

muahahahaha

• Added the Bdubs mecha skin for Armistice. It looks beautiful.
• Added two new NBT tags for the Bdubs entity, pushable and shouldDespawn. Both are booleans.
• Updated showcaseMode. Instead of enabling NoAI and Invulnerable, it now enables Invulnerable, pushable, and disables shouldDespawn. This allows the Bdubs entity to look at the nearby players, which I wanted. I don't believe previous Bdubs entities with this tag will be automatically updated.
• Added new optional altItem field for the Bdubs variant datagen. It's just that, an alternative item for the variant.
• Improved the spyglass waving feature slightly.
• Made it so locked shoulder entities stay locked after relogging.
• Added imposter egg and tnt block items for use in ModFest.