A pure JavaScript implementation of the SHA-256 cryptographic hash function.

This contribution is self-contained and adheres to all project guidelines.

Contribution Checklist:

• Added the pure JavaScript implementation of SHA-256, following the FIPS 180-4 standard.
• Included a detailed README.md that explains the algorithm, its properties, complexity, and provides further reading.
• Wrote comprehensive tests using standard, well-known test vectors to verify correctness.
• All linter checks are passing (npm run lint).
• All tests are passing (npm test -- sha256).