feat: add Substitution Cipher algorithm and tests

[mmohamedkhaled]

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Describe your change:

• [x] Add an algorithm

Checklist:

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized.
• [x] I know that pull requests will not be merged if they fail the automated tests.
• [x] This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] All new JavaScript files are placed inside an existing directory.
• [x] All filenames use the UpperCamelCase (PascalCase) style. There are no spaces in filenames.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or another similar explanation.
• [x] All code is tested using Vitest and follows the ES Module syntax.
• [x] Code is formatted using Prettier.

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Summary:

This PR adds a monoalphabetic Substitution Cipher to the ciphers/ directory.

• Includes substitutionCipherEncryption and substitutionCipherDecryption functions
• Validates key length and character set
• Test coverage includes:
  • Encryption
  • Decryption
  • Non-alphabetic character handling
  • Invalid key rejection
• All tests are passing using Vitest
• Code formatted with Prettier

📚 Reference: Wikipedia – Substitution Cipher