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.tryUseItem to handle ordinary item usage, as well as the event 'TryUseItem' to deal with its failure (e.g. due to Unnerve).
Modify pokemon.useItem to its core part and use it for the moves Pluck, Bug Bite and Fling instead of running directly an event.
Relocate Cheek Pouch effect to 'UseItem'.
This cleans the codebase a bit and fixes a couple of bugs with Cheek Pouch. Namely, its activating when the opponent has Unnerve, and its not activating when using Pluck, Bug Bite or Fling.