Permutation

[Kentwwx]

Describe your change:

Add Permutation, one of the classic questions in backtracking, to the backtracking directory.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Java files are placed inside an existing directory.
• All filenames are in all uppercase characters with no spaces or dashes.
• All functions and variable names follow Java naming conventions.

[siriak]

The code looks good, could you add some tests as in https://github.com/TheAlgorithms/Java/pull/2869/files?

[github-actions]

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[github-actions]

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