Leveraging JSON Schema

The package relies today on its own schema definition in `defineDocumentType` for local content definition.

What I would like to ask if whether JSON Schema can be leveraged instead. The side effect is then that the schema definition is separate from the computed fields which is perhaps a good thing.

```js
import MyJsonSchema from 'myschema/test';

const Doc = defineDocumentType(() => ({
name: 'Doc',
filePathPattern: '**/*.md',
schema: MyJsonSchema,
computedFields: {
....
}
}))
```

The missing piece is that I don't know whether this impacts the architecture.