[heycam/webidl] Define that interface objects are constructors (#698)

ES claims the following (twice):

> Built-in function objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a particular function.

We don't normatively identify the interface object as a constructor, even if it has a `[Constructor]`. I guess we should.

A less obvious question is what we should do if there's no `[Constructor]`. Right now, I think we're aiming to install a [[Construct]] internal method that always throws, but that makes the interface object pass [IsConstructor()](https://tc39.github.io/ecma262/#sec-isconstructor).

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