Add x11-fonts/bpmf-special: Bopomofo IVS boxed and Bopomofo-only fonts

The Bopomofo IVS fonts render a Han character together with its Bopomofo
reading, selected through Ideographic Variation Sequences and Private Use
Area code points. chinese/fcitx5-mcbopomofo 3.0 and later can emit such
text in its Bopomofo font annotation mode, which is aimed at educators and
publishers who need Bopomofo-annotated material.