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.
This allows us to avoid the ugly hack in
`property_accepted_type_ranges()`.
This also updates the `ValueType` to be `opacity-value` rather than
`opacity` to match the spec.