A specified position is now a struct made of two values of different types,
the first one being PositionComponent<X>, and the second one PositionComponent<Y>.
A position component is represented by the new enum PositionComponent<Side>,
with the three values Center, Length(LengthOrPercentage), and
Side(Side, Option<LengthOrPercentage>).
Side keywords are represented by the X and Y enums, which don't include a value
for the center keyword anymore. They are accompanied by the Side trait, which
allows us to determine whether a side keyword is "left" or "top".
This refactor simplified the parsing and serialisation code and exposed bugs in it,
where it would reject valid <position> values followed by arbitrary tokens,
and where it would fail to prefer "left" to "right" when serialising positions
in basic shapes.
To simplify adding additional data to `ParserContext`, this moves test usages to
a few shared locations, instead of being spread across many tests.
MozReview-Commit-ID: 1OahV797eq
