fix: custom child route localization

[BobbieGoede]

🔗 Linked issue

• Prerendering fails when defineI18nRoute is set for nested pages #2351

❓ 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

Resolves #2351

Based on debugging this, I think we may be able to improve this functionality in the future. Currently we expect users to supply a full localized path the a child route using defineI18nRoute or pages in config (e.g. /parent/child), it would be better if users only have to provide localization for the child path segment (e.g. /child or child). This way the parent route could be renamed or the child route could be moved without having to update the custom route localization in multiple files.

📝 Checklist

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

[kazupon]

this PR is great improvement!
LGTM!