Better detection of RequireJS signature

Since CHANNELMGR-1270 and 11.2 back port CHANNELMGR-1530 the Channel Manager does a check for RequireJS having been loaded by a site implementation. It does that by checking a "require" object on the window.

When a client has a "require" object that is not RequireJS, the Channel Manager stops working because dragula.js is not loaded.

Let's make the detection for RequireJS more specific, for instance by checking the number of arguments or (better yet) some internal property.