[heycam/webidl] Switch to multipage ECMAScript spec links (#1007)

It'd make people happy to not have to load the singlepage, and it'd be consistent with how the Bikeshed ecosystem links to the HTML multipage spec.

We should not just prefix links with `/multipage/` because then viewers incur a client-side redirect and the point is to make viewers happier.

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https://github.com/heycam/webidl/issues/1007