[heycam/webidl] How to tell the type of "construct a callback function"? (#557)

I'm wondering if there is a way to describe a signature of [IDL callback function as a constructor](https://heycam.github.io/webidl/#construct-a-callback-function).

We can tell the types of the arguments list for [invoke](https://heycam.github.io/webidl/#invoke-a-callback-function) from the declaration in IDL.
```
callback Foo = void(DOMString arg1);
```
But how can we tell the types of the arguments list for [construct](https://heycam.github.io/webidl/#construct-a-callback-function)?

If there is no way, then I'd like to apply [Constructor] extended attribute to IDL callback functions.
```
[Constructor(DOMString ctor_arg1)]
callback Foo = void(DOMString arg1);
```

FYI, [CustomElementRegistry.define](https://html.spec.whatwg.org/multipage/custom-elements.html#customelementregistry) takes |constructor| of type |Function|, where there is no type information about the constructor arguments.

Explicit type information in IDL is helpful for user agent implementor, I think.


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