Fix Island json serialisation

Island can only JSON-serialise primitive types (e.g. string, number, boolean). If a field has a more complex type, such as Date, it gets automatically converted to a string during serialisation.

This becomes an issue on the client side, where components expect a Date, leading to hydration failures. See this PR for an example: https://github.com/guardian/dotcom-rendering/pull/15622

We need a way to ensure Island can correctly serialise complex types to JSON and reliably deserialise them back into their original types on the client.

**Potential solutions:**
- Use schema validation libraries (e.g. valibot or Zod) to explicitly handle parsing and type restoration
- Use more advanced serialization libraries (e.g. devalue: https://github.com/sveltejs/devalue)