Add support for yaml.marshal and yaml.is_valid

[aron]

This continues on the work done in #100 to add support for yaml.unmarshal. This PR implements the requested changes in that thread.

1. Adds the four yaml and json capabilities to the capabilities.json
2. Ensures the opa test cases are enabled and passing

As part of that I also implemented the other three functions yaml.is_valid, yaml.marshal, json.is_valid to ensure all the test cases pass as well as extending the local test suite.

[srenatus]

Thanks a lot! 👏 🎉