maths: add Kaprekar number implementation

[Eraol]

Describe your change:

• Add an algorithm :
  Add implementation of Kaprekar numbers in maths/special_numbers/kaprekar_number.py.
  A Kaprekar number is a number whose square can be split into two parts that add up to the original number (e.g. 45² = 2025
  → 20 + 25 = 45).

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 Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.

[Raheel-glicth]

can you assign me this work?
@Eraol