Re: [dom] Provide getAttributeNames() which returns an Array<DOMString> (#115)

@annevk Return empty sequence when element has no any attributes is implicitly or should be explicitly defined in prose (like here: https://url.spec.whatwg.org/#dom-urlsearchparams-getall,  https://fetch.spec.whatwg.org/#dom-headers-getall, https://xhr.spec.whatwg.org/#dom-formdata-getall)? 

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