Simple Stackable Potions

Balance: brewing cost now scales with stack size

- The Brewing Stand now consumes 1 ingredient per layer of bottles instead of 1 per cycle. Brewing a full stack of 4 in each slot uses 4 ingredients for 12 potions, the same ingredient-to-potion ratio as vanilla.
- The Brewing Stand won't start brewing if it doesn't have enough ingredients to pay for every layer.
- Potions still stack to 4, and you can still brew whole stacks at once, so you save time but not resources.

Thanks to everyone who gave feedback on the previous behaviour (1 ingredient for 12 potions), which made brewing too cheap.