Re: [webcomponents] Upstream Shadow DOM spec to DOM/HTML Standard (#377)

I want to start working on #57. My tentative plan is:

* Introduce terminology in the HTML standard and later upstream that to DOM. (Terminology as discussed per above, perhaps with adjustments as necessitated.)
* Provide one or more PRs for the cases we know how they should work and open issues against the HTML standard for cases we don't know (e.g., it seems reasonable to open an issue for `currentScript` although I think we probably want to resolve it similarly to `activeElement`).

This will shift some of the development work of Shadow DOM elsewhere, but I think that's okay, since it mostly affects features defined elsewhere. Not too different from the collaboration between Service workers, Fetch, and HTML, or HTML and JavaScript, etc.

I will copy @hayatoito for review. Anyone else who wants to tag along let me know. We could perhaps create a GitHub team called "shadow" that we tag on any such issues and PRs, if that's needed.

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