feat: add Substitution Cipher algorithm and tests

[mmohamedkhaled]

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Describe your change:

• Add an algorithm

Checklist:

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

---

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

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.91%. Comparing base (1d252d7) to head (25add22).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1773      +/-   ##
==========================================
+ Coverage   85.87%   85.91%   +0.04%     
==========================================
  Files         377      378       +1     
  Lines       19720    19779      +59     
  Branches     3005     3017      +12     
==========================================
+ Hits        16935    16994      +59     
  Misses       2785     2785              

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[mmohamedkhaled]

This PR only introduces the Substitution Cipher and its test. I’ve noticed the CI flags code style/spelling issues in files like:

String/AlphaNumericPalindrome.js

Recursive/SubsequenceRecursive.js

Maths/MobiusFunction.js

These issues are pre-existing in the repository and not introduced by this PR. I have run Prettier on my changes, and there are no further modifications I can make to fix these unrelated files.