feat: single file source

[farnabaz]

🔗 Linked issue

resolves #1348

❓ 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

📝 Checklist

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

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[atinux]

I would rename fileName to filename since it is accepted for a while now: https://en.wikipedia.org/wiki/Filename

[xarthurx]

seems this PR has been a while here. Is there any updates for the feature? I came here as I want to query a single file that contains info all in the frontmatter, and it seems the current Nuxt Content's query framework cannot do that...