feat: add perfect number

[ei-sugimoto]

Added perfect numbers with test.

[appgurueu]

The function should be called isPerfectNumber (without the s).

You could optimize it by only iterating up to the square root of n and adding both i and n / i to the sum in each iteration if i divides n (special case: if i is exactly the square root, you don't want to add it twice; I'd deal by this with excluding it from the loop and processing it separately).

Otherwise looks fine.

[ei-sugimoto]

Thank you for the review, I have implemented your suggestion.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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  Files             ?       97           
  Lines             ?     1797           
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  Hits              ?     1730           
  Misses            ?       67           
  Partials          ?        0           

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