Chess Game: Rename nick_name method to nickname for consistency

[lemonade-37]

The problem description uses nickname, but the exercise code defined nick_name.
This update renames the method to nickname for consistency between the description and the code.
Please review.

Exercism Community Topics（Rename nick_name method to nickname for consistency）

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[kotp]

Please link to the discussion on the forum as well.

[lemonade-37]

Please link to the discussion on the forum as well.

I've added the URL for Topics.

[kotp]

I have updated the discussion, waiting on some feedback on direction here. Trying hard to avoid breaking everyone's longtime passing tests for this exercise, if possible, while still doing the right thing.

[kotp]

I am not positive this is all of the files that need to be examined, look in .meta as well.

[kotp]

Looks like discussion in the forum has converged on making all references in code and prose "nickname"! 💪🏾

[lemonade-37]

I have committed a change that renames nick_name to nickname.
Please review.

[kotp]

Thank you for the discussion and the work done!