refactor: separate switchLocalePath SSR feature into plugin

[BobbieGoede]

🔗 Linked issue

❓ 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

I feel like there is a lot going on in the i18n plugin, after looking at Nuxt's source code I noticed some features are separated into plugins, so I tried to do the same here 😅

I also tried another way of reorganizing the plugin logic here #3017, but that's a bit more involved.

If refactors like these actually make things harder to read, do let me know, I'm too familiar with the codebase now to notice if that is the case 😂

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (if possible).
• I have updated the documentation accordingly.

[kazupon]

Good refactoring!
Thanks!