support for `nlohmann::ordered_json` in addition to `nlohmann::json`

[slayoo]

Adding support for the ordered_json type in addition to currently used json would be beneficial for users intending to preserve order of elements in the JSON structures.

For further discussion, see:
https://github.com/nlohmann/json#order-of-object-keys
nlohmann/json#2179
nlohmann/json#2258

P.S. Thank you for a great piece of software (we use it in https://github.com/open-atmos/PyPartMC/)

[slayoo]

@martinRenou @JohanMabille @NoahAmsel @SylvainCorlay ping :)
having ability to use ordered_json would greatly help in our use case
Thanks!

[martinRenou]

Would you like to try a PR? I'm happy to help reviewing 🙂