feat: valid_anagram.cpp

[sgindeed]

Description of Change

This pull request adds a new algorithm for checking if two strings are valid anagrams.

The implementation uses a hash table (std::unordered_map) to efficiently count and compare character frequencies. The file is self-contained and includes Doxygen-style documentation explaining the algorithm, along with a test() function to verify its correctness against various cases.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: Adds a new, self-contained, and tested anagram checker algorithm.

[realstealthninja]

I don't think character frequency could be negative, use an unsigned integer here, fixed width.