Nerf mending books and trading

The mending enchantment is way to easy to get and also way to overpowered. Changes that could make the mending enchantment/book more balanced could be forcing the player to choose between unbreaking or mending, similar to how the mending enchantment works with bows, forcing the player to choose either infinity or mending.

The mending book should not be as easy to obtain. A fix for this would be that it can only be traded by a villager who is at max level, making it impossible to obtain through exploiting the villagers occupation feature by reasigning the villagers occupation until the mending book appears. At the current state you can easily get one of the most overpowered enchantments in your first hour of playing Minecraft.