Re: [heycam/webidl] Need pattern for feature detecting dictionary members (#107)

I had an idea today that I think might actually work. The main sticking point for me is how to avoid exposing the dictionary names to the web; they are currently unobservable, and IMO that is a nice property.

What if we made it easy for dictionary-accepting APIs to get a feature-detection method added to them? I am thinking the following JS code:

```js
createImageBitmap.supports("imageOrientation", "none")
navigator.share.supports("image");
```

These could be generated by Web IDL such as

```webidl
interface WindowOrWorkerGlobalScope {
  [WithDictionarySupports] Promise<ImageBitmap> createImageBitmap(ImageBitmapSource image, optional ImageBitmapOptions options);
}

partial interface Navigator {
    [SecureContext, WithDictionarySupports] Promise<void> share(ShareData data);
};
```

The limitation of this is that it assumes a single dictionary argument. Hopefully that is all we'll ever need... You could imagine generalizations that allow more, but they get uglier to use.

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