fix(module): allow CSS variables in tailwind colors

[davestewart]

🔗 Linked issue

Closes #1986

❓ 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

Right now, Nuxt UI expects Tailwind config to use hex values to describe colors.

However, Tailwind's config is actually more flexible, and allows both RGB channel values and CSS variables.

This PR allows the user to pass values other than hex values:

• HEX are converted to RGB channel values (as before)
• anything else is returned verbatim

This means if the user passes a variable value, it will:

• be proxied to the --color-primary-xxx variables
• be correctly resolved within the 'rgb(var(--color-primary-xxx) / <alpha-value>)' template

If you're happy to merge this, we could do it after #2010 then this could probably be added to the Configuration section.

👀 Note

This PR provides a future opportunity to pass / parse additional config values, such as:

'600': 'rgb(123 50 200)',
'900': 'hsl(245 50 50)',

Whether this is wanted or worth the complication is another matter though!

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly
  • waiting on docs(theming): clarify how nuxt ui augments tailwind config #2010 going through