Enhance environment variables lesson with cross-platform support, validation, and best practices.

[balmukund18]

📚 Enhancement Summary

This PR significantly enhances the Node.js environment variables lesson by adding practical, real-world content that addresses common developer pain points and follows industry best practices.

✨ What's New

🖥️ Cross-Platform Compatibility

• Added Windows Command Prompt and PowerShell syntax alternatives
• Introduced cross-env package for portable npm scripts
• Solved compatibility issues that Windows developers frequently encounter

✅ Environment Variable Validation

• Comprehensive validation examples for required variables
• Type conversion helpers for numbers and booleans with error handling
• Introduction to envalid library for advanced validation patterns
• Practical code snippets that prevent common runtime errors

🔧 Multiple Environment Files

• Explained .env.development, .env.test, .env.production pattern
• Dynamic environment file loading based on NODE_ENV
• .env.example best practices for team collaboration and onboarding

📖 Enhanced Learning Materials

• 5 additional knowledge check questions covering new content
• Updated additional resources with high-quality, relevant links
• Improved practical applicability for real-world development

🎯 Why This Matters

• Addresses real pain points: Cross-platform compatibility is a frequent issue
• Industry standards: Follows 12-Factor App methodology and common patterns
• Practical focus: All additions solve actual problems developers face
• Educational value: Builds meaningfully on existing content without overwhelming beginners

📊 Impact

• +144 lines of valuable content
• 0 breaking changes to existing material
• Enhanced readability and practical applicability
• Better prepared students for real-world Node.js development

🔍 Files Changed

• nodeJS/introduction_to_nodeJS/environment_variables.md - Enhanced with new sections and examples

✅ Checklist

• Content follows The Odin Project style guidelines
• All code examples are tested and functional
• Links are valid and relevant
• Knowledge check questions align with new content
• No breaking changes to existing material
• Maintains beginner-friendly approach while adding depth

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