[dom] Use only lowercase for all ids. (#94)

I see that some of id was changed and using uppercase:

concept-DTL-tokens (was concept-dtl-tokens)
concept-DTL-update (was concept-dtl-update)
concept-NodeIterator-traverse (was concept-nodeiterator-traverse)
concept-traversal-whatToShow (was concept-traversal-whattoshow)

Maybe use one convention (all lowercase letters)?

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https://github.com/whatwg/dom/issues/94