I'm ok with duplication but we should probably bump the major version as well. Not sure for all of packages or not

Ok. I totally agree that @date-io/date-fns should have a major version bump. I don't think we need to bump the version globally.
Should I bump the version(s) myself in this PR ?

We can do that later, but what is requried right now it to ensure that the tests are running for both versions of date-fns and we have proper 100% coverage support

Sorry, why did you remove the date-fns-v2?

Working on the tests, I decided to revert the date-fns-v2 because those have explicit reference to the date-fns package.
And I dont know how to reference two different versions of the same dependency in the same file.

Furthermore, I think that keeping an old version might be overengineered if we do a major version bump on @date-io/date-fns

@dmtrKovalenko do you think we should maintain a compatibility with date-fns v2 on master ?
If so, we might have to change the structure of the repo a little bit to have package-specific tests files.

Thank you for contributation, I am going to merge this right and now and make another breaking changes that I wanted to make for a long time and will release a new version today