Add declarative Shadow DOM features - #5465

The explainer for this feature is here: https://github.com/mfreed7/declarative-shadow-dom/blob/master/README.md
The issue discussion is here: whatwg/dom#831
There is a corresponding Pull Request for the DOM spec that goes along with this PR.

• At least two implementers are interested (and none opposed):

  • Chromium: Implemented and shipped.
  • WebKit: supportive
  • Gecko: supportive

• Tests are written and can be reviewed and commented upon at:

  • In WPT: https://wpt.fyi/results/shadow-dom/declarative?label=master&label=experimental&aligned&q=declarative
  • More tests are being implemented, but the basics are there.

• Implementation bugs are filed:

  • Chrome: https://crbug.com/1042130
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1712140
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=245556

Closes #7069

/acknowledgements.html ( diff )
/document-lifecycle.html ( diff )
/document-sequences.html ( diff )
/dynamic-markup-insertion.html ( diff )
/indices.html ( diff )
/infrastructure.html ( diff )
/parsing.html ( diff )
/scripting.html ( diff )