Update InspectorXSLT to handle Metapath not XPath

[wendellpiez]

User Story:

A current limitation of the InspectorXSLT is that its underlying implementation limits the use of @target in constraints as documented here: https://github.com/usnistgov/metaschema-xslt/wiki/InspectorXSLT#metapath-implementation. Only a subset of XPath (not fully contiguous with Metapath) currently works correctly.

All solutions to this problem eventually go through Metapath.

Goals:

Given #58, we can

• Mock up a mini XSLT parsing XPath 3.0 paths using iXML in Saxon
  • Support with XSpec tests showing routine constructs
• Migrate this XSLT into generation pipeline
• As appropriate, provide InspectorXSLT with mitigations for known Issues (see Some complex but valid Metapath in constraint targets do not produce functional Inspector XSLTs. #91)
• Given a Metapath grammar, migrate syntax support to Metapath as supported by a Metapath grammar

Dependencies:

#58 is a dependency.

A Metapath grammar in iXML is a dependency.

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.

{The items above are general acceptance criteria for all User Stories. Please describe anything else that must be completed for this issue to be considered resolved.}