filesystem: JsonEditor: offer interactive retry on error
When json.loads() fails (e.g. the user made a syntax mistake), prompt the user with "Re-open file for editing? [y]es/[n]o:" and loop back to the editor if they answer 'y' or 'yes'. If the user declines, return the original unmodified value so no write is attempted; the temp file is still cleaned up by __exit__() in that case.
