Patching JSON data in data packs

Currently, it is possible to modify things such as loot tables or world generation in data packs. However, this requires overriding the entire data for what you want to modify. For example, if Data Pack A wants to add tin ingots to the loot table for dungeon chests and Data Pack B wants to add portal lighter for a dimension it adds to the same loot table, then one of the data pack's changes to the dungeon loot will be lost. I propose adding functionality to modify JSON data without completely overriding it, so you could, for instance, add an entry to a loot table for your own items or remove entries for items you don't want.

* for everything defined in JSON, except maybe languages and tags, since language and tag files can already modify existing data without overriding it altogether