fix: check process.cwd before calling it

[kraenhansen]

🔗 Linked issue

Fixes #148

❓ 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

Some runtimes (like React Native) has a process global, but doesn't provide a cwd function. Instead of assuming the cwd function is available, just because the process global is, merging this PR will update the check to actually verify the process.cwd is a callable function before relying on it.

Resolves #148

📝 Checklist

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