ENHANCED: Add support for relaxed XSD date/time literal notation.

[wouterbeek]

This adds support for the relaxed XSD date/time literal notations that are already documented for rdf_canonical_literal/2, specifically:

• date/3
• date_time/[6,7]
• month_day/2
• year_month/2
• time/3

This fixes the issue reported in https://github.com/SWI-Prolog/swipl-devel/issues/282