feat: add initial tw4 support in v6

[ineshbose]

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

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[what-the-diff]

PR Summary

• Integration of 'addVitePlugin' import from '@nuxt/kit'
  The team introduced the import addVitePlugin from the @nuxt/kit. This added functionality assist in enhancing the plugin handling capability of the application.

• Inclusion of Tailwind CSS Version 4 detection mechanism
  A new check was introduced to identify if Tailwind CSS version 4 is being used. If detected and the experimental supports are not enabled, the system will now provide a warning to the user.

• Modification of 'resolveCSSPath' for Tailwind CSS Version 4 support
  The resolveCSSPath was modified to support Tailwind CSS version 4. If the resolution fails, it has been designed to generate a default CSS file automatically for Tailwind CSS 4.

• Conditional addition of Vite plugin for Tailwind CSS
  New logic was added to add the Vite plugin for Tailwind CSS only under specific conditions: if the builder is @nuxt/vite-builder and Tailwind 4 is detected.

• Upgrade of PostCSS plugin handling
  The PostCSS plugin handling system was updated. If Tailwind CSS version 4 is detected, the system will support it, otherwise it will revert back to the previous configurations.

• Extension of 'ExperimentalOptions' type
  The ExperimentalOptions type was expanded to include a new tailwindcss4 option. This will allow users to leverage the capabilities of Tailwind CSS 4 within the application.

[rowild]

I am confused. How can I tell my Nuxt application to use tailwindcss 4? Just simply by changing the @tailwind directives to import @tailwind?