Items recipes content

This pull request contains two parts:
1. I added some content to content/items.dart; a bottled potion for every element.
2. I added some coin recipes to content/recipes.dart; It might be a useful mechanic to forge 3 coins of the same tier into 1 coin a tier above. Maybe it doesn't make strict RP sense (I don't think you can forge Silver from any amount of Bronze) but could be useful to the game?