Add declarative shadow trees

A longer explanation of this feature can be found at https://github.com/mfreed7/declarative-shadow-dom/blob/master/README.md. It was originally discussed in whatwg/dom#831.

Corresponding DOM PR: whatwg/dom#892.

Tests: shadow-dom/declarative in WPT with web-platform-tests/wpt#42833 being the latest PR as of this commit.

Closes #7069.

Author: mfreed7