Provide a way to maintain old element ids

[jyasskin]

Is your feature request related to a problem? Please describe.

w3c/sustainableweb-wsg#338 (comment) reports that some ID attributes changed, which might break existing inbound links.

Describe the solution you'd like

Bikeshed allows an oldids attribute on heading elements, to cause the old IDs to point there. Presumably Respec will use data-oldids instead.

[xfq]

FWIW, we use things like this currently: https://github.com/w3c/clreq/blob/69874f726bc3b49ae34c5f3ad42c9d20dc912ee4/index.html#L322