[dom] DOMTokenList validation for object creation and attribute updates (#106)

@yoavweiss I think we forgot to cover this since the text wasn't present yet in the `DOMTokenList` section but instead sorta left to each individual element/attribute combination.

Do you agree that we should update https://dom.spec.whatwg.org/#interface-domtokenlist to validate the tokens coming from attribute sets?

(See the two paragraphs before the domintro box.)

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/106