Migrate XML access and manipulation to new Moxml gem

[ronaldtse]

We have another effort called Moxml that unifies the interface for XML API:

• https://github.com/lutaml/moxml

Currently, it works with Nokogiri and Oga. Support for Ox is coming soon.

We want to move lutaml-model to use Moxml, and move any lutaml-model XML code in there.

This allows us to easily switch XML adapters, and also gives the user a consistent API to write custom methods for accessing/manipulating XML content.