Added Sales Tax Calculator

[arjun-shettigar1605]

Fixes Issue🛠️

Closes #1729

Description👨‍💻

This project is a web-based sales tax calculator that allows users to compute the gross amount and tax amount based on the net price and tax rate, or to determine the tax rate based on the net and gross amounts. It also includes a pop-up information window as a user guide.

Type of Change📄

• [x] New feature (non-breaking change which adds functionality)

Checklist✅

• [x] I am an Open Source contributor
• [x] I have performed a self-review of my code
• [x] My code follows the style guidelines of this project
• [x] I have commented on my code, particularly in hard-to-understand areas

Screenshots/GIF📷

Popup guide :)

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