feat: add warning for when node polyfill is used in non-node environments

[pi0]

🔗 Linked issue

❓ 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

node-fetch-native is intended to be used with proper export conditions otherwise a build with node-fetch polyfills will be used in non-Node.js environments like workers and can be silently happening too.

This PR adds an explicit warning to guide users:

Node.js compatible build of node-fetch-native is being used in a non-Node.js environment. Please make sure you are using proper export conditions or report this issue to https://github.com/unjs/node-fetch-native. You can set process.env.DISABLE_NODE_FETCH_NATIVE_WARN to disable this warning.

📝 Checklist

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