improve reliability of the browser/node check

this fixes compatibility with a popular GTAV modification framework, FiveM. they define the window object as an alias for global, so the check was always returning true, causing various problems.

Author: vecchiotom

src/util/Constants.js

@@ -1,6 +1,6 @@
 const Package = exports.Package = require('../../package.json');
 const { Error, RangeError } = require('../errors');
-const browser = exports.browser = typeof window !== 'undefined';
+const browser = exports.browser = typeof process === 'undefined';
 
 /**
  * Options for a client.