[whatwg/dom] [proposal] Node.containingShadowRoot (#826)

DRANK

I think this would be useful for authors that want to write Shadow-DOM aware code. All browser engines have a way to get a hand to the subtree root and check if it's a shadow root in one way or another. Gecko: https://searchfox.org/mozilla-central/rev/2e355fa82aaa87e8424a9927c8136be184eeb6c7/dom/base/nsINode.cpp#699 WebKit: https://trac.webkit.org/browser/webkit/trunk/Source/WebCore/dom/Node.cpp?rev=+254087#L1127 Chromium: https://source.chromium.org/chromium/chromium/src/+/master:third_party/blink/renderer/core/dom/node.cc;l=1815;drc=95e78e2d51fce9616e316073be6b2b92189e21ee How do folks feel about this? Maybe it should just be in `Element` and `Text` or something... cc @whatwg/components