json-ld expand/compact improvements

We have some code to handle the basics, but ideally we would use a library for compact/expand.

jsonld.js works, but is large and doesn't work that well with rollup:

• I've created a test repo: https://gitlab.tugraz.at/vpu-private/Playground/json-ld-experiments
• I've created an issue with jsonld.js and asked for help: https://github.com/digitalbazaar/jsonld.js/issues/355