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.
For example, in the following abbreviated test HTML:
<span>some text</span>
<script>println("whf")</script>
We would have to craft the expectation file to include the "some text"
segment, usually with some leading whitespace. This is a bit annoying,
and makes it difficult to manually craft expectation files.
So instead of comparing the expectation against the entire DOM inner
text, we now send the inner text of just the <pre> element containing
the test output when we invoke `internals.signalTextTestIsDone`.
