Fix: switch from placeholder: to data-placeholder: prefix class for SelectValue

[damienroche]

🔗 Linked issue

close #866

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ 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

On radix-vue docs, placeholder is set by using [data-placeholder] attribute. To apply style, we need to use data-[placeholder]: prefix class instead of placeholder: prefix which is used only for input type

📸 Screenshots (if appropriate)

📝 Checklist

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