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.
Represent WebIDL C++ types with a single CppType model that tracks
nullability, optional presence, and contained storage.
GC-like values now use GC::Ref/GC::Ptr directly, while containers choose
"plain", "Root", or "Conservative" container types depending on what
they contain. For example, sequence<Element> becomes a RootVector of
GC::Ref values, while sequence<SomeDictionary> becomes a
ConservativeVector only when the dictionary contains GC-like values.
This moves the generated bindings away from wrapping GC values in
GC::Root by default.
This has broad fallout as the types passed to interfaces for GC
objects changes almost fully across the board.
After looking into this more, the `[[declarations]]` slot does not seem
to need to be a literal map of property names and values. Instead, it
can just point at the source (an element or style declaration), and
then direct all read or write operations to that.
This means the `has()` and `delete()` methods actually work now.
A few remaining failures in these tests are because of:
- StylePropertyMap[ReadOnly]s not being iterable yet
- We're not populating an element's custom properties map, which get
fixed whenever someone gets around to producing proper computed
values of them.
