Move stringifyValues utility to core

[taylortom]

`stringifyValues` is a generic pure function with no API-specific logic. It is being moved to `adapt-authoring-core` so that consumers don't need an api dependency just for serialisation.

This change removes the local implementation and re-exports `stringifyValues` from core for backwards compatibility.

[taylortom]

Change added to existing PR #72 on branch issue/71 instead.