We now expand shorthands into their respective longhand values when
assigning to a shorthand named property on a CSSStyleDeclaration.
We also make sure that shorthands can be round-tripped by correctly
routing named property access through the getPropertyValue() AO,
and expanding it to handle shorthands as well.
A lot of WPT tests for CSS parsing rely on these mechanisms and should
now start working. :^)
Note that multi-level recursive shorthands like `border` don't work
100% correctly yet. We're going to need a bunch more logic to properly
serialize e.g `border-width` or `border` itself.
