use cross-env for platform-independent scripts

[CreatorLZ]

Fix: Add cross-env for universal script execution

Problem:
The current dev and prod scripts, which use the NODE_ENV=... syntax, are not compatible with Windows command prompts (e.g., cmd.exe or PowerShell). This results in the error: 'NODE_ENV' is not recognized as an internal or external command, operable program or batch file.

This prevents Windows-based developers from easily running the project in development or production mode as intended.

Solution:
This PR introduces the cross-env package as a devDependency and updates the dev and prod scripts to use it.

• "dev": "cross-env NODE_ENV=development node index.js"
• "prod": "cross-env NODE_ENV=production node index.js"

cross-env is a standard, lightweight solution for setting environment variables in a way that works on all major operating systems, including Windows, Linux, and macOS.

Benefits:

• Improved accessibility: The project becomes more accessible and easier to contribute to for developers on different platforms.
• Portability: The package.json scripts are now truly cross-platform, which is a best practice for open-source projects.
• No breaking changes: This change is fully backward-compatible and has no negative side effects on the project.