feat: Add nuxt file extension configuration

[zhaolinlau]

🔗 Linked issue / Discussion

#118

❓ Type of change

• [x] 📖 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)
• [x] ✨ 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

Since not all the Nuxters are using TypeScript, including me, so the Nuxtr extension should also support the .js file extension. And this PR added an option for them to choose either .ts or .js for file creation, and it's .ts by default.

📝 Checklist

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