stanzajs missing subMultiExtension

Hi,

I'm trying to understand what happened to subMultiExtension from this library when it got moved to here: https://github.com/legastero/stanza/blob/master/src/jxt/Types.ts

Is there some other way I can get the same functionality?

The code looks like

```js
fields: {
   test: types.subMultiExtension(NS, 'test', OtherType);
};
```
