Add SulfurCube#equipItem

[Strokkur424]

This PR exposes the NMS-internal SulfurCube#equipItem method as an API-method.

Currently, if you want to set the item swallowed by a SulfurCube, you have to call the following code:

cube.getEquipment().setItem(
  EquipmentSlot.BODY,
  ItemType.BIRCH_LOG.createItemStack()
);

This is fine if you know about it. However that skips a little bit of logic that's inside the NMS SulfurfCube#equipItem method. Additionally, it's not intuitive for anyone simply scrolling through the SulfurCube API methods.

(Discussion: https://discord.com/channels/289587909051416579/925530366192779286/1526310872450994318)

[mbax]

If it isn't just a shortcut to editing the equipment I'm not sure I like it being called equipItem. Especially with the potential dropping of another item. I want some more thoughts on naming. I like SulfurCube#swallow(ItemStack)

Alternatives I thought of:

• Engulf
• SwallowAndMaybeBarf
• Nom

[Doc94]

Well its based in the nms method maybe just a tryEquipItem but the original name is fine to me... (maybe can mention the events for what this method can pass?)