[heycam/webidl] Define a callback constructor type (#701)

In particular something we verify to be a callback that also passes https://tc39.github.io/ecma262/#sec-isconstructor.

This could be used to remove the first step of https://html.spec.whatwg.org/multipage/custom-elements.html#dom-customelementregistry-define.

It would also help https://github.com/WebAudio/web-audio-api/pull/1843 and other Worklet APIs dealing with constructors.

Invoking the constructor would go through https://heycam.github.io/webidl/#construct-a-callback-function though that can be tightened up a bit by no longer having to check that it's a constructor.

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