[heycam/webidl] Should make it clear that [Global] and [Constructor] can't coexist (#744)

Today I saw this in a spec draft:
```
[Exposed=AnimationWorklet, Global=AnimationWorklet, Constructor
 (optional any options
)]
interface StatelessAnimator {
};
```
which is nonsense: `[Constructor]` means you can create the things, and `[Global]` means there is only one of it per Realm, and it's the global.  We already have prose saying:

> The [Global] extended attribute must not be used on an interface that can have more than one object implementing it in the same Realm.

but we should explicitly disallow using `[Global]` and `[Constructor]` together.

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https://github.com/heycam/webidl/issues/744