fix: use legacy property publicPath for the assets url.

[wattanx]

🔗 Linked issue

Fixes: #635

❓ 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

In nuxt 2 the option to set the cdn url is publicPath.
https://v2.nuxt.com/docs/configuration-glossary/configuration-build/

The app.cdnURL is not used directly and must be set to publicPath.

Also, currently NUXT_APP_CDN_URL is not supported and must be set to app.cdnURL.

export default defineNuxtConfig({
  app: {
    cdnURL: process.env.NUXT_APP_CDN_URL
  }
})

📝 Checklist

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