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.
For us, that's KeywordStyleValue and CustomIdentStyleValue.
CustomIdentStyleValue now has a .cpp file to reduce the number of
includes in its header file.