Ad-hoc terms use DefinedTerm/Class/Property

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http://schema.org/DefinedTerm is better than http://schema.org/Thing for pre-existing URIs that are semantic artefacts or concepts. Somewhat equivalent to skos:Concept. This will then be consistent with https://www.researchobject.org/ro-crate/1.2-DRAFT/profiles that also mentions http://schema.org/DefinedTermSet.

For simplification with the profiles, rdf:Property / rdfs:Class types now optional, so we can stay in schema.org land for lightweight properties using their equivalent http://schema.org/Property and http://schema.org/Class. The RDFS variants are needed to do rdfs:subPropertyOf or rdfs:subClassOf but not I think needed for just adding a few terms to JSON-LD.

The part about @context and ad-hoc terms will still need to be updated to show that a RO-Crate Profile Crate could be used directly as a @context to use its terms there.

Also fixed some bugs in the expanded RDFS code from #208 (as @id is needed for object references in our JSON-LD).

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Rendered at https://stain.github.io/ro-crate/1.2-DRAFT/appendix/jsonld#add-local-definitions-of-ad-hoc-terms

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Merging without review as call 2023-03-23 agreed to merge all outstanding PR (that are not WIP) so we can do an overall review and push out 1.2.