Improve the item_model Component in Java

Please make the item_model component more user-friendly for larger resource packs. The new items folder is a great new feature, but it would be better for a more template-like feature. Instead of custom_model: "item_name", consider a structure like custom_model: {"model": "namespace:path/to/model", "behavior": "namespace:file_in_items_folder"} with:

• assets/namespace/models/item/path/to/model.json
• assets/namespace/items/file_in_assets_folder.json.

This would make adding new items to the game much easier (QoL), and it would also shrink resource packs by a bit, which is good for servers.