feat: add option to not use @vitejs/plugin-legacy

[wattanx]

🔗 Linked issue

Fixes: #1126

❓ 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 like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

I want to add an option not to use @vitejs/plugin-legacy.
I believe that having the option not to use @vitejs/plugin-legacy would make it easier to migrate to Nuxt 3 using vite mode.
(I think it will be a big change, so I decided to opt-in.)

📝 Checklist

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