Move PropertyNameAndID, custom property data, registered custom
properties, and Typed OM associated property names to Utf16FlyString.
This removes the FlyString storage boundary from CSS property-name
handling and lets CSSStyleProperties keep the name it receives from
CSSOM instead of converting it back to UTF-8.
The "subdivide into iterations" part is left as a FIXME for now, until
we have a way of knowing if a property is a list or not.
The parse_a_css_style_value() helper has an unwieldy return type because
of the requirement that it return either one value or a list of values,
but sticking to the spec here seems worthwhile for now.
