Added function to check happy number with testcases

[TaranjeetSinghKalsi]

Added a function to check if a given number is a happy number or not. Also added and improved testcases.

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• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
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