Declare os to prevent installation on non-Windows systems

[TrevorBurnham]

I have a package that conditionally requires registry-js on Windows only. However, it's installed in node_modules regardless of platform.

This inefficiency could be addressed if registry-js declared in its package.json that it only runs on Windows:

"os": ["win32"],

Then consumers could declare it in optionalDependencies and it would only be installed on Windows devices.

Important: This would be a breaking change requiring a new major version, because npm would throw an error on non-Windows devices if this package is declared as a non-optional dependency.