feat: Use toJSON for otherwise-empty objects

[hashtagchris]

deepMap currently returns empty objects ({}) for common types like Date and URL. This PR updates deepMap to check for a toJSON custom serialization function, and uses that to return a string representation of the object.

References

JSON.stringify also uses toJSON.

[hashtagchris]

Going to evaluate whether to always call toJSON if present, or just improve the serialization of instances of Date and URL specifically.