`Jsonify<unknown>` should be either `unknown` or `JsonValue`

Assuming you're serializing a non-strictly typed JSON data, all of its descendents were serialized too, thus `JsonValue` seems like a reasonable assumption. But resolving to `unknown` would be enough too.

Currently:
```ts
type U = Jsonify<unknown> // never
type V = Jsonify<{foo: unknown}> // {}
type W = Jsonify<[unknown]> // [null]
```

I am OK to submit a PR if you're OK with it