fix: provide nuxt useRoute and useRouter composables

[danielroe]

This reverts commit 5e5fa50.

🔗 Linked issue

resolves #554, nuxt-modules/color-mode#169

❓ 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

When we switched to using native vue-router composables in #533, we lost support for using useRoute and useRouter in Nuxt plugins, middleware, etc. (ie. anywhere outside of a component). This PR reverts that change in that if called outside a composable it will revert to previous behaviour. However, within a component, we still use the native vue-router composable.

📝 Checklist

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