Align with new WebIDL constructor syntax

[pmdartus]

Recently, the WebIDL spec replaced the [Constructor] extended attribute by a constructor operation (WebIDL spec PR). We are currently in the process of updating all the depending specification to use this new syntax (HTML spec PR, DOM spec PR).

[ExE-Boss]

I didn’t realise that WebIDL2JS still uses the legacy [Constructor] extended attribute while working on jsdom/jsdom#2737.