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, #2812