Allow <ruby> in phrase-level environment

[747]

There is <ruby> element (with <rb>, <rt>), which is a general-purpose inline structure mostly used in East Asia, around in TEI since P5 4.2.0.

Could we have it as a phrase-level element? It is quite versatile while seems to be one of a kind in the TEI schema, so I don't know how much change should be made to incorporate it in Lex-0 model, but I would be able to answer any question regarding modeling details.