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.

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.16%. Comparing base (e21aee8) to head (b1994b9).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6943      +/-   ##
============================================
+ Coverage     78.15%   78.16%   +0.01%     
- Complexity     6503     6507       +4     
============================================
  Files           741      741              
  Lines         21675    21680       +5     
  Branches       4236     4238       +2     
============================================
+ Hits          16940    16947       +7     
+ Misses         4063     4062       -1     
+ Partials        672      671       -1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.