feat(module): add theme.defaultVariants option

[benjamincanac]

🔗 Linked issue

Resolves #2934

❓ 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

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[nuxthub-admin]

✅ Deployed ui3

Deployed ui3 fac8fe0 to preview

🔗	pr-2934.ui-6q2.pages.dev
📌	71d599d5.ui-6q2.pages.dev
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[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@4400

commit: fac8fe0