Teach the Rust parser to recognize declarative shadow root templates and
pass the parsed mode, slot assignment, clonable, serializable, and
focus-delegation flags to the C++ DOM host.
Expose shadowRootSlotAssignment reflection with the spec-defined named
missing and invalid value defaults, and extend the ShadowDOM text test
coverage for the reflected property and parser-created shadow roots.
Specification: https://dom.spec.whatwg.org/#concept-event-dispatch
This also introduces shadow roots due to it being a requirement of
the event dispatcher.
However, it does not introduce the full shadow DOM, that can be
left for future work.
This changes some event dispatches which require certain attributes
to be initialised to a value.
