Add value type for free form data

[pkedy]

The value WIDL type would be a useful for for use cases where the a schema should include free form data. This would be similar to a JSONValue types in various JSON libraries. It would be up to the format library (e.g. as-msgpack, tinygo-msgpack) to have the correct idiomatic types for encapsulating an objects, array, string, numbers, boolean, etc and how the developer determines what is enclosed.