Implemented Complex Number Multiplication in the String folder

[muskansngh07]

Description:

This PR introduces a utility class ComplexNumberMultiplication in Java that multiplies two complex numbers represented as strings.

Features:

• Accepts two complex numbers in the format "a+bi" and "c+di".
• Returns the product as a string in the same format "x+yi".
• Uses the formula: (a + bi) × (c + di) = (ac - bd) + (ad + bc)i.
• Handles positive and negative integer parts.
• Includes a main method.

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
• [x] All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java