[dom] Add caniuse info boxes like the HTML spec (#69)

The HTML spec displays little info boxes with relevant caniuse data next to the definitions of various entities.
caniuse likewise has data for many DOM features. It would be nice if the DOM spec could display this data similar to the way it's done in the HTML spec.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/69