Make the fix for #71 backwards compatible so we can publish this fix with "only" a minor version-number increase

As per suggestions of @sidorares in
* https://github.com/marc-portier/json-bigint/commit/04723e0a9023f93b677f37c4a708288c07d63fcb#commitcomment-7397586 and
* https://github.com/marc-portier/json-bigint/commit/04723e0a9023f93b677f37c4a708288c07d63fcb#commitcomment-7397623