[webidl] add a type to represent the undefined value (#60)

It's too awkward to deal with the undefined value by having to use `any`. Let's introduce a new type (`undefined`?) to represent that one value. Some care will be needed to make sure we handle optional arguments, distinguishability, etc. properly.

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Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/60