About Pull Request

This PR adds long backgammon gameplay and statistics. Actually it adds general backgammon core with one of possible shell.

Successful PR check-list

[X] I explained what's changed.
[X] The PR is ready to merge and completely finished (or I've written out what I'm going to do next in this PR and moved it to draft).
[ ] I've requested a review from the project owner or some of the contributors.
[X] I've read contributing guide.
[X] I understand that the PR will not get a review until all checks have been passed.
[X] I understand that I can't merge PR without a review.