Add ReturnOnInvestment to Maths

[InukaWijerathna]

Description

Adds returnOnInvestment to the Maths/ directory.

Return on Investment (ROI) is a core financial formula that measures how profitable an investment is relative to its cost:

ROI = (Gain - Cost) / Cost × 100

What's included

• Maths/ReturnOnInvestment.php — clean typed function with PHPDoc, throws \InvalidArgumentException for non-positive cost

Checklist

• Follows the code style of the project (typed parameters, PHPDoc block)
• Throws \InvalidArgumentException for invalid input
• Strict types enforced

Reference: https://www.investopedia.com/terms/r/returnoninvestment.asp

[InukaWijerathna]

Closing this for now — needs more work on my end before it's ready.