feat: tailwind-variants

[Sandros94]

Following the work on Nuxt UI v3, migrate to tailwind variants.

Elements

• [x] dark-mode (sort of, will wait till ui3)
• [x] links
• [x] links-group
• [x] headline
• [x] headline-span

Layouts

• [x] card
• [x] footer
• [ ] header
• [x] main
• [x] page

Units

• [ ] figure
• [ ] hero

[Sandros94]

I'm probably going to do this progressively, while creating new components or editing existing ones