Add return on investment to maths

[InukaWijerathna]

Description

Adds returnOnInvestment to the maths/ directory.

Return on Investment (ROI) is a straightforward financial formula that measures how profitable an investment was relative to what you put in:

ROI = (Gain - Cost) / Cost × 100

What's included

• maths/return_on_investment.dart — typed function with doc comment, throws ArgumentError for non-positive cost, includes a main() with example outputs

Checklist

• Follows the code style of the project
• Throws ArgumentError for invalid input
• Includes main() with sample outputs

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

[InukaWijerathna]

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

[InukaWijerathna]

Hi! Just checking in to see if there's any update on this PR — happy to make any changes needed to get it merged. Thanks for your time!

[akashgkrishnan]

Add tests.

Add the question.

Use named variables for readability
Handle edge cases