Support brewing recipes in tags

[tal5]

Additions

• BrewingRecipe class - a temporary class to hold an input, ingredient, and result of a brewing recipe. Can be replaced by Paper's one once Spigot support is dropped.
• ItemHelper#getCustomBrewingRecipes - returns a Map of keys to brewing recipes.
• ItemHelper#getCustomBrewingRecipeIDs - returns a Set of all brewing recipe keys.
• ItemHelper#getCustomBrewingRecipe - returns a brewing recipe by ID.
• ItemHelperImpl#getNMSCustomBrewingRecipes - returns the internal custom brewing recipe map
• ItemHelperImpl#paperMixToRecipe - converts paper's PaperPotionMix into a BrewingRecipe.

Changes

• ItemTag.recipe_ids, server.recipe_ids, server.recipe_items, server.recipe_type, and server.recipe_result now support custom brewing recipes on Paper servers.
• Updated some code to modern java.
• Cleaned up imports.

Notes

• The code is slightly messy currently because we can't access Paper's object directly, but I tried setting it up in a way that would make it easy to clean up later once we go Paper-Only.
• The reason for the separate methods is to avoid going over the entire map converting to BrewingRecipe's with reflection for simple things such as getting the IDs. We could maybe remove them once we go Paper-only and that code is cleaned up - Let me know if these methods should be merged already.