[heycam/webidl] What is "missing" special value? (#794)

For optional arguments, we refer a special value "missing" in the spec.  But in common programming languages, it is not common to have similar features.
Regarding to the conversion rules, It is convertible with an ES value **undefined** ([Overload Resolution algorithm](https://heycam.github.io/webidl/#es-overloads) and [Callback Inteface](https://heycam.github.io/webidl/#es-user-objects)).  It means "missing" is very similar to an IDL value Undefined.  What is the difference between "missing" and Undefined, and should we explicitly differentiate them in bindings code?

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https://github.com/heycam/webidl/issues/794