If available uses ISOcodes::ISO_3166 to validate country code.
Otherwise assume that the code is valid as long as it is a two-letter code or
__. See rr_publisher_country() for use of country codes.
validate_country_code(country)[character(1)]
a two-letter country code or AA, ZZ or
__ (special country codes for SHERPA/RoMEO)
TRUE if the country code is valid, errors otherwise