feat: add neon number implementation

[Nithwin]

Description of Change

Added a C++ implementation of the Neon Number algorithm in the math directory.
The implementation includes checking if a number is a "Neon Number" (where the sum of digits of its square equals the number itself).

Key features:

• Implemented is_neon function in math namespace.
• Added full Doxygen documentation.
• Included self-tests using cassert to verify functionality (tested with 0, 1, 9, 12, 45).

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
• 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.