[Amendment: Alexander <protonull@protonmail.com>]
PlayerTradeEvent is used for player purchases from villagers and wandering
traders, but not custom merchants created via Bukkit.createMerchant(). During
discussions in Discord it was decided that it'd be better to add a new event
that PlayerTradeEvent inherits from than change getVillager()'s annotation to
@Nullable, especially since that'd also infringe on the implication of the
event being about villager trades.
