[docs] TypeSpec JSON Interoperability Documentation

Since the langauge and emitter ecosystem is maturing it would be useful to have a prescriptive specification of how to encode values representing TypeSpec data types to JSON for interoperability with other emitters.

This would allow us to easily answer questions like:

- What is the normative representation of `TypeSpec.bytes` in a JSON payload?
- What are the _must use_ decorators that need to be checked when emitting a model property, union, or other such type that may need special attention when encoding as JSON?
- How do I choose a JSON representation for a given scalar type?

Adherence to such a specification wouldn't be a hard requirement _per se_, but doing so would ensure that JSON serializations are compatible between different emitters, and it would serve as a reference for emitter authors.