Every line of the String array would be an item + difficulty to drop the item + chance + mode

• item: item id
• amount: amount
• difficulty to drop the item: the amount of difficulty required for the item to drop, works differently based on mode
• chance: chance for the drop to happen
• mode:
  • minimum: will try to drop the item when the difficulty matches or is higher (default)
  • multiple: will try to drop the item everytime difficulty is a multiple of required difficulty.
• chance_mode:
  • flat: chance is the percentage chance for the item to drop if the difficulty criteria matches (default)
  • stacking: each point of difficulty >= 'difficulty to drop the item' will be multiplied by the chance (e.g. chance 2% and difficulty 10, difficulty required 5, chance to drop the item will be chance * (difficulty - difficulty_required + 1) = 2% * (10 - 5 + 1) = 12%)

By default nether star shards will be

"progressivebosses:nether_star_shard,1,2,2,MINIMUM,STACKING", 
"progressivebosses:nether_star_shard,2,4,4,MINIMUM,STACKING", 
"progressivebosses:nether_star_shard,4,8,8,MINIMUM,STACKING"

2% chance per difficulty >= 2 to drop 1 shard + 4% chance per difficulty >= 4 to drop 2 shards + 8% chance per difficulty >= 8 to drop 4 shards