Move StylePropertyMap, StylePropertyMapReadOnly, and the CSS.supports
property-name overload to Utf16FlyString. Request the UTF-16 binding
conversion path for their IDL property-name arguments.
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.
Previously, some StyleValues created a large number of intermediate
strings during serialization. Passing a StringBUilder into the
serialization function allows us to avoid a large number of these
unnecessary allocations.
This inheritance exists for typed-om classes, but StyleValues aren't
typed-om.
Somehow this makes our z-index interpolation slightly more correct. 🎉
A couple of arbitrary substitution functions require us to get or
produce some style value, and then substitute its ComponentValues into
the original ComponentValue list. So this commit gives CSSStyleValue a
tokenize() method that does so.
Apart from a couple of unusual cases like the guaranteed-invalid value,
style values can all be converted into ComponentValues by serializing
them as a string, and then parsing that as a list of component values.
That feels unnecessarily inefficient in most cases though, so I've
implemented faster overrides for a lot of the basic style value
classes, but left that serialize-and-reparse method as the fallback.
