add permutation

[cpiao3]

Describe your change:

• [ x] Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

References

Checklist:

• [ x] I have read CONTRIBUTING.md.
• [ x] This pull request is all my own work -- I have not plagiarized.
• [ x] I know that pull requests will not be merged if they fail the automated tests.
• [ x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [ x] All new Java files are placed inside an existing directory.
• [ x] All filenames are in all uppercase characters with no spaces or dashes.
• [ x] All functions and variable names follow Java naming conventions.
• [ x] All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• [ x] If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[cpiao3]

@siriak can you review this; added combination as well.

[siriak]

The code looks good, please also remove main() and add tests as in https://github.com/TheAlgorithms/Java/blob/ab9f74c2ee0dee9174616f2154f1161d600f68cd/src/test/java/com/thealgorithms/maths/KaprekarNumbersTest.java

[cpiao3]

@siriak added tests and deleted main()

[siriak]

LGTM, thanks!