refactor: restructure extend vue-i18n

[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

We still have some split logic between this module and what was originally in vue-i18n-routing, because of this we were extending vue-i18n in two places.

After some reorganizing I noticed that onExtendExportGlobal and onExtendVueI18n were actually doing the same thing but with different variable names 🤔 possibly __composerExtend did the same as well, after merging and generalizing these extends it appears to still be working.

I feel like my changes should have broken something.. 😂 Either the refactored code was interchangeable or it contains untested logic 🤔

📝 Checklist

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

[kazupon]

Good Refactoring!
Thank you always! ❤️