[css-ruby] ruby-position on 2 line subtitles/captions

Recent work on supporting deployed Japanese Subtitling/Captions in TTML
indicates a requirement to support ruby positioning on 2 line
subtitles/captions where the first line uses right/above (before) and the
second line uses left/below (after). We have addressed this in TTML by
introducing an 'outside' keyword, which is interpreted as 'before' for
lines 1 through N-1 and 'after' for the Nth (last) line.

In TTML, we opted to use 'before' and 'after' as writing mode relative
values (rather than right/left, above/below). So we ended up with three
possible values: before | after | outside.

We found the same requirement applies in the use of text emphasis marks (圏点),
so have specified the same three values for text emphasis position style.