Align / update Metapath support under XSLT M4

[wendellpiez]

User Story:

The XSLT M4 pipeline integrates (experimental, provisional) support for parsing Metapath syntax. Currently, this support takes the form of a stovepiped XSLT-based XPath parser produced using an open-source engine over an unofficial grammar.

More lately @david-waltermire-nist has been framing Metapath support in Java, based on a leaner and tighter grammar.

In XSLT M4, Metapath parsing is used primarily to cast XPath addressing XML-based OSCAL (as given in metaschemas, i.e., Metapath expressions), to XPath addressing JSON-based 'object description' OSCAL (i.e. matching XML elements in the XML expansion of JSON produced by commodity tooling), for use inside M4 implementations of (a) JSON to XML conversion; and (b)
the Metatron validator i.e. the implementation of Metaschema constraints (using Metapath-based matching and testing) under XSLT M4 by way of programmatic generation of Schematron.

Current functionality in these pipelines needs to be preserved, but based on the new (smaller) grammar, many improvements could be made to (most) all handling of Metapath internally.

Goals:

Improve support for these features in XSLT M4, for integrity (do not accept bad inputs) and robustness.

Dependencies:

Consider doing this alongside unit testing for Metaschema-based constraints checking. Also continue maintaining the current unit testing of functions, which should (now) show failures, for example, where now-disallowed constructs appear. Unit testing should be aligned with any external testing of Metapath.

• Updating the XSLT-oriented grammar is a precondition (Remove any unwanted node tests from Metapath #52)

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.

[aj-stein-nist]

No attaching it to new or existing milestone. Come back to it later when we review later in the year to determine priorities and work scheduling.

[wendellpiez]

This Issue is vague, but amounts to "stabilize and align Metapath across implementations".

Since current XSLT M4 implementation is stable/static but behind (see usnistgov/metaschema-xslt#12), it amounts to the same as others respecting constraints support (#43, usnistgov/metaschema-xslt#13), namely its specification and unit testing.

Assuming those are proceeding, this is closable.