implement rarity

items in the bingo should be picked by their rarity/probability. This opens up new possibillities for the gm as epic/legendary items can be much harder to get - therefor much better and much more useful!

No files, tests, or entry points are named. Start by locating the bingo item-selection logic and clarify how rarity tiers and probabilities should be represented before coding. Done means bingo items are selected according to agreed rarity probabilities, with epic and legendary items appearing less often.

Written by the indexing model from the issue text.