[heycam/webidl] Investigate why no [[HasInstance]] is defined for named constructors (#282)

I'm aware that [[HasInstance]] is deprecated, yet it's defined for interface objects.

The following is the observed behavior in FF and Chrome:

```js
new Image() instanceof HTMLImageElement;
// => true
new Image() instanceof Image
// => true
```

The latter especially doesn't seem to be explained by what's in the spec.

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