wip: add schemaorg serializer

[mfenner]

Description

Initial implementation for a schemaorg JSON serializer. Works in principle, but a few optional properties need more work.

Checklist

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
• I've added relevant documentation.
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• I've followed the user interface guidelines (for relevant code).
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• I've NOT included third-party code (copy/pasted source code or new dependencies).

Third-party code

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[mfenner]

@zzacharo I labelled this as draft, as I still want to make some changes. Sorry for not communicating clearly. Should be done today, or can do a separate pull request.