Move the remaining CSSStyleDeclaration property-name APIs to
Utf16FlyString. This lets CSSOM binding and generated accessor code
pass JS property names without first constructing FlyString values.
Keep internal custom-property and descriptor storage unchanged for now.
Those remaining FlyString conversions are at storage boundaries that
will be migrated in follow-up commits.
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.
