This wins us 65 new WPT subtest passes! It also shows up that we're
doing the wrong thing in ShorthandStyleValue in places, notably with
the grid properties. However, having one place to fix instead of two
will make it easier to correct them. :^)
In order to be fully correct, we should use the algorithm here:
https://drafts.csswg.org/cssom/#serialize-a-css-value
However, it's quite hand-wavy. What we do have in the meantime is
`ShorthandStyleValue::to_string()`, where we special-case the
serialization rules for shorthands with a generic fallback that's
equivalent to what the previous `get_property_value()` code was doing.
Better support for CSS shorthands when setting the style attribute. This
improves some tests in WPT /css/css-align/default-alignment/*shorthand.
When setting the style attribute in JS via element.style = '..' or the
setAttribute method, shorthand properties were not expanded to longhand
properties.
