added mortgage-calc

[Suswetha6]

Fixes Issue🛠️

#1863

Closes #

Description👨‍💻

Added a mortgage calculator with appropriate html,js and css.

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📷