[w3c/webcomponents] Step 13.3-4 in element definition should clarify the ordering (#542)

Steps 13.3 and 13.4 of the [element definition](https://html.spec.whatwg.org/#dom-customelementsregistry-define) algorithm doesn't explicitly state the ordering of `Get` other than implicitly defining it via a map.

We should either call it an ordered map, or explicitly state the order by which these properties are obtained as they're observable via `Proxy` objects.

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