Add CodeMeta terms

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As agreed in RO-Crate call 2023-09-28, we will add the additional CodeMeta terms to the RO-Crate context.

Some considerations before we merge this:

• CodeMeta 3.0 from July is not quite released yet -- see Next release? codemeta/codemeta#236 (comment)
• @dgarijo has established https://w3id.org/codemeta and https://w3id.org/codemeta/3.0 as PIDs for their context (see Choose identifier for the CodeMeta schema codemeta/codemeta#321)
  • as usual we'll anyway avoid nested context resolution and have our own fixed declarations directly in our context
• Their namespace https://codemeta.github.io/terms/ is not persistent as it relies on github.io (Codemeta content negotiation is not correct codemeta/codemeta#216)
• Terms like https://codemeta.github.io/terms/readme are 404 Not Found, which we don't really want RO-Crate users to deal with.
  • This is not helped by there not being any ontology or vocabulary beyond their JSON-LD context. We don't need that, but at least the link should go to something.
  • @dgarijo has proposed using https://w3id.org/codemeta# as namespace, which I support. But this may mean waiting for a 4.0 release. Possibly a 3.1 can be done quicker, as 3.0 is not publicly announced as far as I can see.
• Schema.org terms maintainer and funding are redefined by Codemeta (see Switch funding to schema.org  codemeta/codemeta#274 and Write proposal for the schema.org adoption of the CodeMeta terms codemeta/codemeta#232)
  • I've commented out these two, as our context-generator luckily bails out on conflicts

Given these uncertainties I am reluctant to add this directly, but I've prepared a branch we can keep ready. I've commented (perhaps too much!) in these issues to perhaps help move things forward.

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Ping @CaroleGoble who wanted a list of issues!