feat(theme): add 'gruvbox-light' theme

[mertssmnoglu]

Description of change

I heard that flawz need some more themes #6. I tried something to implement a Vim Light Theme called gruvbox-light It's NOT EXACT SAME but it's looks like gruvbox-light.

How has this been tested?

It can only be tested with the eyes. Testing in daylight is recommended.

Color Palette of gruvbox-light

---

If we can agree on theme colors, I will add them too.

[orhun]

Very nice! I like it.

Can you also update README.md with a screenshot too?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.

Project coverage is 7.84%. Comparing base (6698b7c) to head (68f3b6c).

Files	Patch %	Lines
src/theme.rs	0.00%	27 Missing ⚠️

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Additional details and impacted files

@@           Coverage Diff            @@
##            main     #32      +/-   ##
========================================
- Coverage   8.14%   7.84%   -0.30%     
========================================
  Files         13      13              
  Lines        712     739      +27     
========================================
  Hits          58      58              
- Misses       654     681      +27     

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[mertssmnoglu]

I changed the colors of the selected line so that it can be highlighted more easily.

Also updated README.md with theme preview. Other previews are 1000x600 but I couldn't take the screenshot at the same resolution.

[orhun]

I updated the screenshot with the same resolution :)

Thanks for the contribution!