Add nightfox, dayfox, and terafox themes

[EdenEast]

Description

This pr adds nightfox, dayfox and terafox from the colorscheme EdenEast/nightfox.nvim

Nightfox

Dayfox

Terafox

Theme Submission Checklist

• [x] Included theme in iTerm2 format
• [x] Included 600x300 screenshot, 13pt Monaco font, no transparency
• [x] Updated README.md with new theme and screenshot
• [x] Updated CREDITS.md with new theme
• [x] Ran tools/gen.py to generate themes in all formats
• [x] Updated screenshots/README.md with new theme