type definition does not match code for `registerWidget`?

**Describe the bug**

_I'm willing to submit a PR to address this, but want to be sure I'm not misunderstanding something first..._

The `CMS.registerWidget()` [source](https://github.com/netlify/netlify-cms/blob/dbf2920254fb3682e12463a6df8ded4b94b55be0/packages/netlify-cms-core/src/lib/registry.js#L86) takes four arguments:

```js
export function registerWidget(name, control, preview, schema = {}) { ... }
```

but the [typescript interface exported for it](https://github.com/netlify/netlify-cms/blob/dbf2920254fb3682e12463a6df8ded4b94b55be0/packages/netlify-cms-core/index.d.ts#L565-L569) doesn't include the `schema` argument

```
registerWidget: (
widget: string | CmsWidgetParam,
control?: ComponentType | string,
preview?: ComponentType,
) => void;
```

Additionally, the actual `registerWidget` function checks whether its first argument is an array, and if it is calls its self on each member of the array, but the typescript interface does not recognize this usage as valid.