[dom] Add "inclusive siblings" term (#55)

Selectors defines `:nth-child()`/etc in terms of index among siblings, and it's intended that if the element doesn't have a parent its the only sibling (so `:first-child` and `:last-child` both match, etc).  I need a term for "an element + its siblings".

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/dom/issues/55