Move JSON to its own library

Description / Motivation

See !41 (comment 1559390488)

To summarize: in Octez there is code that uses the JSON module of Tezt without actually needing to depend on anything else from Tezt. And depending on Tezt just to use the JSON module is a problem because once linked with Tezt, Tezt tries to parse the command-line.

Since the JSON module is useful on its own, let's separate it in its own library tezt.json.

(This MR also removes a duplicate entry from the changelog, and adds a missing entry.)

How to Test the MR Manually

dune build && dune runtest (or trust the CI)

Checklist

• Update CHANGES.md. No need to document changes to documentation and tests.
• Make sure all new values, types etc. are documented in .mli files and that the generated documentation looks ok.
• Add tests in test/, if relevant.