Improve Markdown/up conversions (phase 1) XSLT M4 data conversion

[wendellpiez]

User Story:

As a user of OSCAL conversion tools I need my Markdown conversion to be dependable, consistent and correct. XSLT M4-based conversion tools are now showing a bug cf usnistgov/OSCAL#1151.

Goals:

• Following on usnistgov/metaschema#213, address problems exposed in test-suite/metaschema-xspec/markdown-types/markdown-mix.xspec
• Stabilize and build out Markdown/markup conversion testing with view to testing other implementations
• Optionally: instrument or demonstrate how the tests can be applied not only to the core converter, but a (generated) format converter such as an OSCAL JSON-to-XML converter or a converter generated for a mini metaschema

Dependencies:

None, but note additional Acceptance Criteria, including one for OSCAL integration.

Acceptance Criteria

• [ ] markdown-mix.xspec passes; so do other XSpecs now working, with appropriate modification.
• [ ] readme files are updated as needed
• [ ] All website and readme documentation affected by the changes in this issue have been updated. Changes to the website can be made in the docs/content directory of your branch.
• [ ] A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• [ ] The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.
• [ ] The OSCAL build works successfully and OSCAL converters can be shown to work correctly with the updated Metaschema M4 XSLT implementation

[wendellpiez]

This Issue needs to be picked up in the context of Markdown conversion unit tests.

Since that is a dependency for this Issue (at least end-to-end) should we plan on integrating that?