Tidy up JEI integration, add RegistryAccess to ZGatherHints interface

Minor changes to improve the JEI integration. Mainly wanted to make it possible to get the sources and javadocs for JEI by using its real maven instead of cursemaven. The addition of the `ZGatherHints#getRegistryAccess()` method matches the old `ZGatherHints` and is useful for https://github.com/VazkiiMods/Quark/pull/5045