Field subscription `modifiedSinceLastSubmit`, is defined as `false`, rather than `boolean`.

Hello there,
In file `index.d.ts`, `modifiedSinceLastSubmit` is defined as:
```
export interface FieldSubscription {
...
  modifiedSinceLastSubmit?: false;
...
}
```
Any idea why it is `false` rather than `boolean`? Is this intentional?

Thanks