Node.Pretty() and Node.PrettyPrint()

Need to add Node.Pretty() (json string) and an equivalent Node.PrettyPrint() in order to free testing from specific Node.Type constants to facilitate grammar development. It will still be JSON, just a more more verbose version of it. This can also be used for better error messages when needed. The Node.String() and Node.JSON() are still authoritative and should always be used when working with multiple ASTs from different sources.