maths/special_numbers: add Kaprekar number

[Kcstring]

Describe your change

Add implementation of Kaprekar numbers to maths/special_numbers/kaprekar_number.py.

A Kaprekar number is a non-negative integer whose square can be split into two parts that sum back to the original number. For example:

• 9: 9² = 81, and 8 + 1 = 9
• 45: 45² = 2025, and 20 + 25 = 45
• 297: 297² = 88209, and 88 + 209 = 297

This implements OEIS A006886.

Reference

• https://en.wikipedia.org/wiki/Kaprekar_number
• https://oeis.org/A006886

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.
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