Add navbar funtionality & fix multiple errors.

[marsianjohncarter]

1. Added file navbar.js which contains the JS functionality
2. Fixed errors with GSAP library by:
   • Moving static library imports to the top of index.html.
   • Putting all GSAP funtionality inside the DOMContentLoaded event listener in script.js.
3. Fixed/changed navbar styles in navbar.css.
4. Moving the nav element inside index.html.

hacktoberfest

[marsianjohncarter]

156

@divyanshudhruv , could you do your magic?

[divyanshudhruv]

Merged. Maggiicc!