Add return on investment to maths

[InukaWijerathna]

Description

Adds returnOnInvestment to the maths/ directory.

ROI is one of the most widely used financial metrics — it tells you how profitable an investment is relative to what you spent:

ROI = (Gain - Cost) / Cost × 100

What's included

• maths/return_on_investment.ts — typed implementation with JSDoc, throws RangeError for non-positive cost

Checklist

• Follows the code style of the project (arrow function export, JSDoc, RangeError for invalid input)
• Type annotations for all parameters and return value

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

[InukaWijerathna]

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