Improvements to the JsonValue type

The `JsonValue` type was thrown together rather quickly to support glTF extras and not really given a lot of careful consideration. We should make sure it's an interface we're happy to commit to long-term.

We should consider:
* Is it ever ok to throw exceptions? When?
* Is it important to have an integer type distinct from a double type? What about signed versus unsigned? What about big integers?
* Is our interface clear, easily discoverable, and hard to misuse? Look at https://github.com/nlohmann/json for interface inspiration, though we probably don't need or want to support that entire feature set.
* Is the performance and memory usage acceptable?