[webcomponents] Add dfn.js (#402)

https://resources.whatwg.org/ hosts a useful tool called dfn.js that lets you figure out where a particular defined term is referenced from. This makes it easier to find algorithms referenced through different names and makes it overall clearer how globally important something is.

I would love to see this added to Shadow DOM.

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/webcomponents/issues/402