With this commit, only direct CSSStyleValues created from internal
StyleValues can be converted back to a StyleValue. More subtests will
pass as create_an_internal_representation() is implemented for the
various CSSStyleValue subclasses. :^)
Gets us... a LOT of WPT passes, because there's a ton of coverage for
each property.
After looking into this more, the `[[declarations]]` slot does not seem
to need to be a literal map of property names and values. Instead, it
can just point at the source (an element or style declaration), and
then direct all read or write operations to that.
This means the `has()` and `delete()` methods actually work now.
A few remaining failures in these tests are because of:
- StylePropertyMap[ReadOnly]s not being iterable yet
- We're not populating an element's custom properties map, which get
fixed whenever someone gets around to producing proper computed
values of them.
