Added Calculator with improved UI and features

[kiruba-r11]

Description

Calculator built using HTML, CSS, JS, and Bootstrap added. It is responsive and its UI and features have been improved.

Fixes #168

Type of change

• This change requires a documentation update

Checklist:

• My code follows the style guidelines(Clean Code) of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests/screenshots(if any) that prove my fix is effective or that my feature works