Convert Vigenère Cipher to Node.js Module

Description:
Refactor the existing vigenere.js script to work as a reusable Node.js module that can be installed via npm and imported in other scripts. This makes the cipher logic modular and scalable.

Expected Tasks:

• In js-ciphers/, create a proper Node.js module:
  • Export encrypt and decrypt functions
  • Use module.exports or export syntax
• Add a package.json with name, version, description
• Create a sample test.js to demonstrate how to use the module
• Update the root README.md with:
  • Installation instructions
  • Example usage in Node.js

Stretch Tasks:

• Publish it locally via npm link
• Add a TypeScript version with types included