Make villager trading affected by difficulty

Newly spawned villagers change the items they can sell depending on the difficulty.

An example I came up with for librarians

Hard → Do not sell Mending enchantment

Normal → Sell Mending enchantment only in swamp biome

Easy & Peaceful → Have a chance to sell Mending enchantment in all biomes (as before)

This will make the difficulty have more impact on the gameplay, and you will be able to play the game that suits your playing style. Players who felt that the villager trading was OP, and those who liked the villager trading as before, will be satisfied with this change.