Tag hierarchy support

Firstly, I would like to thank you for your great work with this package. I have been using it for a while now and I sincerely find the query syntax to be very intuitive.
Regarding the issue at hand, I have noticed that org-ql's tag search mechanism (particularly `org-ql--tags-at`) does not take hierarchies (i.e., tag groups) into considerations. I understand that tag hierarchies are not that common and their syntax may be unnecessarily complicated (e.g., regular expressions) but, nevertheless, I think they are a useful thing to have.