Item Attribute Modifiers

This update adds some stuff to make configuring Frostiful with datapacks a little easier, with a particular eye toward mod packs.

• Added a new datapack file category called 'item attribute modifiers'
  • This datapack category is found under data/[namespace]/frostiful/item_attribute_modifiers.
  • It allows a datapack to specify different attribute modifiers for the 'generic' version of any item.
  • See wiki for specific format
• Added new temperature effect type: frostiful:empty
  • This does literally nothing, its purpose is to make it easier to overwrite other effects
• Fixed some issues with icicles