Add support for imported types

[ExE-Boss]

This only requires that the imported module exports the validate (and eventually expose) method.

Unlike https://github.com/jsdom/webidl2js/pull/109, this works cross‑realm by not using instanceof on the wrappers.

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Fixes https://github.com/jsdom/webidl2js/issues/69 Supersedes and closes https://github.com/jsdom/webidl2js/pull/109