Implement the `prax` gem as a replacement for Jing

[ronaldtse]

Jing (https://github.com/relaxng/jing-trang) is a validator of XML documents and accepts schema inputs in the RELAX NG and RNC formats.

The rng gem performs parsing and building of RELAX NG and RNC schemas.

The task is to implement XML validation that Jing allows in the prax gem.

[ronaldtse]

These notes provide a good basis for XML validation:

• https://relaxng.org/jclark/implement.html