Add RDFLib Path to SHACL path utility and corresponding tests

[mgberg]

Summary of changes

This adds a utility to take an RDFLib-friendly representation of a property path and build the corresponding SHACL Path representation in RDF (e.g. the objects of triples with predicate sh:path). The provided path object could either be:

• A URIRef, when the path is simply a predicate
• A Path, when the path is complex (e.g. a/rdfs:subClassOf*)

This makes it easy to create complex SHACL paths from RDFLib without needing to remember the details about how to construct the RDF representations of those paths correctly.

This is sort of a follow-up to #2699 which added the opposite functionality.

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.256% (+0.009%) from 90.247%
when pulling 2163692 on mgberg:build-shacl-path
into a9e30d3 on RDFLib:main.