Filling gaps in XSLT-M4 Metaschema support

[wendellpiez]

Committer Notes

This PR is planned to address at least:

• Refactoring 'empty' data type as assembly with no contents Support empty assemblies instead of an 'empty' datatype #49 Empty Required Flag Handled Differently XML vs. Json #66
• Implementing ANY functionality Implement and test 'ANY' feature in XSD and JSON Schema #117 Current biblio model in json schema is unusable. OSCAL#792
• Valid root notes Generated JSON Schema does not enforce root element #11 Extend to support multiple designated (valid) root nodes #31 Current jsonschema objects (and documentation) appear to have elements which are not in the document definition. OSCAL#814

Potentially also addressing documentation issues.

Note: these are breaking changes for the OSCAL metaschemas, which will have to updated to reflect changes in the metaschema model. (Suggest reviewing for ANY usage at the same time, with placement of root-name.)

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you squashed any non-relevant commits and commit messages? [instructions]
• Do all automated CI/CD checks pass?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you included examples of how to use your new feature(s)?
• Have you updated all website](https://pages.nist.gov/metaschema) and readme documentation affected by the changes you made? Changes to the website can be made in the website/content directory of your branch.

[wendellpiez]

@david-waltermire-nist this is ready to test. (I will also continue to test locally.) But do we want to pull into an integration branch?