Setting the Unlock Challenge

Every backpack has the option to set an unlock challenge. Unlock challenges are simply a goal the player can complete in order to unlock new backpacks. They are inspired from Advancements from the vanilla game, except the main difference is that the player unlocks a reward (the backpack). Unlock challenges are persistent per world, they do not transfer to different saves, just like Advancements. Partial completion of unlock challenges is also saved (e.g. if the player has to cut down one-hundred oak logs, and they have only cut fifty, Backpacked will remember fifty oak logs have already been cut).