Update parsing support with iXML

[wendellpiez]

User Story:

Metapath support must go "all the way down" to a grammar. We need an iXML version of this grammar so as to provide processing support in XProc/XSLT-based pipelines.

https://invisiblexml.org/ - Invisible XML
https://coffeepot.nineml.org/ - Java implementation composable in XProc (Norm Tovey-Walsh)
https://github.com/nineml/abnf2ixml - parsing support for ABNF-defined grammars
https://github.com/johnlumley/jwiXML - iXML in-browser demo and test (John Lumley)

Reasons for iXML here:

• quasi-standard / externally specified
• well supported in our established stack (XProc/XSLT)
• we can unit test it and applications using it
• more or less easily aligned with IETF ABNF? see above

Goals:

• mini demo
• permanent home / maintenance plan for Metapath iXML grammar

A proof-of-concept iXML demonstration would offer a pipeline that would consume arbitrary iXML-defined inputs (i.e., inputs conformant to given grammars), both at end points and internally, including within XSLT (e.g., support via a function library).

Such a proof of concept might exist (we could ask); or we can offer such a PoC back to the XProc/XML community.

A Metapath implementation would be the application of such a pipeline using a putative Metapath grammar, in a useful way. The application does not have to be complete for this Issue to be complete. We do however need a permanent home and/or strategy for maintaining and testing the grammar going forward.

With such an application in hand, a follow-on issue would be the replacement of the current parsing support.

Dependencies:

A grammar or grammars for testing, including a (draft/demo) grammar for Metapath, either in iXML format or convertible (expressible) in iXML.

A normative specification for that grammar (even if only sketch/draft/wip) where we can align with community on Metapath semantics.

Acceptance Criteria

• 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.
• At least one new Issue is created to plan and execute switching the old path-parsing support in Metaschema XSLT with new/better iXML-based path parsing aligned with a formal specification for Metapath