DSL: add `tags` stanza

only limited/defined keys are permitted
 - :vendor
 - :font_pitch
 - :font_serif
 - :font_weight
 - :font_slant

forward-compatibility: no associated functionality is
provided.  Intentionally left undocumented.

References: #4688
Merge pull request #4953 from rolandwalker/add_tags_stanza

DSL: add `tags` stanza