This isn't part of the CSSOM API directly, but will be used by
StylePropertyMapReadOnly.has() to avoid doing the work to serialize a
string version of the property's value, just to throw it away again.
This reverts 0e3487b9ab.
Back when I made that change, I thought we could make our StyleValue
classes match the typed-om definitions directly. However, they have
different requirements. Typed-om types need to be mutable and GCed,
whereas StyleValues are immutable and ideally wouldn't require a JS VM.
While I was already making such a cataclysmic change, I've moved it into
the StyleValues directory, because it *not* being there has bothered me
for a long time. 😅
I was wrong when I added those notes before about this being impossible,
it's *very* possible, for example with the `@page margin` descriptor.
However, until we have a large number of these shorthands and not just a
single example, we can get away with hard-coding support for it.
This holds the boilerplate that's needed by any CSSStyleDeclaration
subclass that holds Descriptors. CSSFontFaceDescriptors now only has to
worry about initialization and its own exposed properties.
