Added Playfair cipher (JavaScript) in Cryptography Algorithms

What:
Adds implementation of the Playfair cipher in JavaScript, including encrypt and decrypt functions. Includes README explanation and unit tests.

Why:
The repository already includes classical ciphers (Caesar, Rail-Fence, Hill). The Playfair cipher is a historically important digraph substitution cipher and completes this cryptography section.

How:

• playfairCipher.js contains the core logic (Key matrix creation, pair creation, pairwise encryption and decryption)
• README.md explains the algorithm, how to use the code and sample input/output
• playfairCipher.test.js includes tests for typical and edge cases

Testing:
Run npm test — all tests should pass, including new ones.

Checklist:

• Code follows the style of existing algorithms in the repo
• Explanations/documentation added
• Tests added and passing
• No breaking changes