feat: add `loadLocaleMessages` to manually load locale messages

[BobbieGoede]

🔗 Linked issue

• 740

❓ 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)
• [x] ✨ New feature (a non-breaking change that adds functionality)
• [ ] ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Resolves #740

Adds the function loadLocaleMessages which allows users to load locale messages for non-loaded locales when using lazy-load translations.

📝 Checklist

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

[nuxt-studio[bot]]

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