Add support for RFC8819 "module-tag" extension

RFC8819 defines a YANG extension for specifying module tags. Teach parser to recognize and report it.

Unlike similar extensions, we also need to create a dedicated type matching the typedef and deal with default IANA registry (https://datatracker.ietf.org/doc/html/rfc8819#section-7.2) values in a manner which makes sense. Some thought needs to be put into how that Tag class will interact with the encapsulation class created by Binding for that same typedef.