Handle reliance on 'window' object as optional

[haywirez]

Right now, the package fails validation when trying to use in node apps or service workers where window is not available:

Uncaught ReferenceError: window is not defined
    at index.js:33:72 in node_modules/gltf-validator/gltf_validator.dart.js
    at index.js:15:50 in __require2
    at index.js:20881:17

Would be nice to address this if possible! My use case involves server-side validation.