feat: add infix to postfix, prefix, and prefix to infix expression converters

[Sreekar-Reddy-D]

Description

Added three expression conversion algorithms:

• infix_to_postfix.cpp
• infix_to_prefix.cpp
• prefix_to_infix.cpp

Each file:

• Uses assert-based unit testing
• Fully documented with Doxygen comments
• Placed in strings/ folder per CONTRIBUTING.md

Checklist

• Code compiles and runs
• test() function included
• Matches CMU and repo style guidelines

[realstealthninja]

Open seperate prs or merge all the functions to one class

[Sreekar-Reddy-D]

->Merged all conversion functions (infix_to_postfix, infix_to_prefix, prefix_to_infix) into a unified expression namespace
->Removed unnecessary class wrapper for simplicity
->Added detailed Doxygen-style comments and modular helper functions (is_operator, precedence, etc.)
->Improved readability and maintainability without changing existing logic
->All test cases pass successfully

[Sreekar-Reddy-D]

As per your suggestion to either open separate PRs or merge all conversion functions into a single class, I have chosen the latter approach.

I have merged all the conversion functions (infix_to_postfix, infix_to_prefix, prefix_to_infix) into a unified expression namespace.

The previous class wrapper was removed to simplify the structure and improve clarity.

I also added modular helper functions such as is_operator and precedence, along with Doxygen-style documentation for better maintainability and readability.

All existing logic remains unchanged, and I have verified that all test cases pass successfully.

Please let me know if any further changes are needed.