Renamed is-hidden to have prefix, using kind

Oops, forgot to mention that our `is-`like modifiers should have `nb-` prefx too, so they won't have possible conflicts with any other projects.

Also, I think at least for the few releases we should have the old `_hidden` class to have hidden styles, so if someone've used it, they won't lose it. But we should encourage them to use the new proper class, as we would remove it in the future.

Also, using stylobate kind for hiding.