feat: added new feature i.e text decrypter

[tanujbordikar]

Description

UNDER GSSOC'23

This PR's code provides a collection of functions for performing encryption and decryption using various ciphers: Caesar cipher, Affine cipher, Substitution cipher, and Transposition cipher. Users can choose the type of cipher and input their desired text and key values for encryption or decryption.

Fixes #457

Have you read the Contributing Guidelines on Pull Requests?

• [x] Yes
• [ ] No

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update
• [ ] Documentation Update

Checklist:

• [x] My code follows the style guidelines(Clean Code) of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I've edited the SCRIPTS.md and link to my code.
• [x] I have created a helpful and easy to understand README.md
• [x] My documentation follows Template for README.md
• [x] My changes generate no new warnings
• [ ] I have added tests/screenshots(if any) that prove my fix is effective or that my feature works.