Enhance Vigenère Cipher to Support Numeric Characters in Encoding and Decoding

[SaeeBiwalkar]

This pull request updates the Vigenère cipher implementation to support numeric characters during both encoding and decoding processes.

Changes Made:

• Character Set: The alphabet now includes numeric characters (0-9) alongside the alphabetic characters (a-z).
• Encoding Logic: Adjusted the encoding logic to correctly handle numeric characters, ensuring they are encrypted in accordance with the Vigenère cipher rules.
• Preservation of Case: The implementation continues to preserve the case of alphabetic characters while leaving numeric characters unaffected.

Benefits:

• Broader Usability: Users can now encrypt and decrypt strings containing both letters and numbers seamlessly.
• Enhanced Functionality: This enhancement improves the utility of the Vigenère cipher for modern applications that often involve alphanumeric data.
  Please review the changes and consider merging them to enhance the functionality of the Vigenère cipher operation. Thank you!