Support referencing TS type definitions for `type: 'json'` fields

In some cases where you're using `type: 'json'` you'd like to reference some existing TS type definitions instead of the `any` type that's currently used for `json` fields. This applies to both regular `fields` and `computedFields`.

Example:


Rough API idea:

```ts
{
// ...
typescriptType: { name: 'IReadTimeResults', from: 'reading-time' }
}
```

## Notes

- A potential other direction/idea for this could be to "derive" nested document types based on existing TS types. (Would address #149)