Support writing to OSCAL Classes

[ronaldtse]

In order to transform AsciiDoc into OSCAL content.

Coradoc-OSCAL will convert the content of the resulting AsciIDoc into OSCAL.

Dependencies:

• [ ] https://github.com/metanorma/coradoc/issues/1
• [ ] https://github.com/metanorma/coradoc/issues/2
• [ ] https://github.com/metanorma/coradoc/issues/13
• [ ] https://github.com/metanorma/coradoc/issues/9
• [x] https://github.com/metanorma/coradoc/issues/10

[ronaldtse]

Once this is done we can move some code to https://github.com/metanorma/coradoc-oscal/issues/1