fix: do not use require.main in cli preset

[jrutila]

🔗 Linked issue

Part of implementing #2466

❓ 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

The generated server/index.mjs is no longer dependent on the require. I don't think there is a valid way to check if the file is the main with ES modules?

Also, it uses Object.entries for result headers.

📝 Checklist

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

[pi0]

Would you please rebase it to v2 branch?

Also trick for ESM main check: https://github.com/unjs/unenv/blob/main/test/node-coverage.ts#L75

[jrutila]

Uhh... sorry, need to fix more...