Item usage refactor
Deprecate the method pokemon.eatItem, as well as the events 'Eat' and 'EatItem', due to being fundamentally the same as pokemon.useItem, 'Use' and 'UseItem'.
Implement the method pokemon.useItemSuccess  to handle extraordinary item usage (Pluck, Bug Bite, Fling). It's also called by pokemon.useItem.
Implement the event 'TryUseItem' to deal with the failure of ordinary item usage (e.g. due to Unnerve).
This cleans the codebase a bit and fixes bugs with Cheek Pouch (its activating when the opponent has Unnerve, and its not activating when using Pluck, Bug Bite or Fling), and Symbiosis (not activating when an ally eats a berry).