feautre: json export, PR proposal

[benedekfazekas]

I am working on a tool that would visualise pgn-s, specifically variations but using different lang/stack so i looked into exporting a pgn into json using python chess. this is what i came up with:
https://github.com/benedekfazekas/python-chess-json-export-sandbox/blob/main/your_cli_tool/json_exporter.py
as I am doing the above tool in clojure it can also do edn

would you accept this in a PR?

[Mintas]

I am not the maintainer of python-chess, but I think this would be a nice feature, but I suppose that to consider implemetations there should be also:

1. deserializer to import data from json
2. open PR with changes