[css-ruby] collapse ruby-position values

I'm not sure if this has been proposed before, but there seems no need for
four values {over, under, left, right} when two would suffice: {before,
after}, where 'before' means prior to (before) the first edge in the line
progression direction and 'after' means subsequent to (after) the second
edge in the line progression direction:

*WM          'before'     'after'*
lrtb        over         under
rltb        over         under
tbrl        right        left
tblr        left         right