[api-minor] Add a (static) PDFWorker.getWorkerSrc method that returns the current workerSrc

This method returns the currently used `workerSrc`, which thus allows obtaining the fallback `workerSrc` value (e.g. when the option wasn't set by the user).

src/display/api.js

@@ -1485,6 +1485,10 @@ var PDFWorker = (function PDFWorkerClosure() {
     return new PDFWorker(null, port);
   };
 
+  PDFWorker.getWorkerSrc = function() {
+    return getWorkerSrc();
+  };
+
   return PDFWorker;
 })();