A few improvements for generating tags

[nikita488]

1. There is a method called copy (and copyEntry) inside vanilla ItemTagsProvider which is used for generating Item tag from Block tag. So it will be useful to have this method inside RegistrateTagsProvider.
2. It will be cool to have a way to automatically generate Item tags when you register a block with Tag using BlockBuilder. Basically, when you attaching the tag to a block, you will want the same tag for item (for use in recipes, etc) and right now you need to manually copy every block tag to item tag.

[tterrag1098]

Added part 1 in 0100b79