[vector-api] Use firstElementChild/nextElementSibling in XML parsers

This PR uses [`firstElementChild`](https://developer.mozilla.org/en-US/docs/Web/API/ParentNode.firstElementChild) and [`nextElementSibling`](https://developer.mozilla.org/en-US/docs/Web/API/ParentNode.nextElementSibling) to speed up parsing of XML documents.

Note that these properties are not supported on `Document` elements (except in very recent browsers), hence the separation of `ol.format.KML#readNameFromDocument`.

Thanks to @fredj for the tip-off.