Until now, we've been using CSS::LengthPercentage, sometimes wrapped in
Optional, to represent CSS sizes.
This meant we could not support modern values like `min-content`,
`max-content`, `fit-content(<length>)`. We were also conflating `none`
and `auto` which made the `min-*` and `max-*` properties confusing.
The new CSS::Size class covers all possible size values as individual
substates. It'll be quite a bit of work to make all layout code aware of
the additional features, this patch merely makes the new type available.
