Added C++ Implementation

[Rajabhianu]

This PR adds a C++ class for the Vigenère cipher, supporting both encryption and decryption of alphabetic text.

Features:

• Encryption: Encrypts alphabetic characters using a key, preserving case and non-alphabetic characters.
• Decryption: Reverses the encryption to recover the original plaintext.
• User Input: Accepts plaintext and key from the user and displays the encrypted and decrypted results.

Testing:

• Manually tested with various inputs to ensure correct functionality and preservation of non-alphabetic characters.

[CLAassistant]

All committers have signed the CLA.