[api-minor] Add support for PageMode in the API (issue 8657)

Please refer to https://wwwimages2.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/PDF32000_2008.pdf#page=82.

src/core/worker.js

@@ -770,6 +770,10 @@ var WorkerMessageHandler = {
       }
     );
 
+    handler.on('GetPageMode', function wphSetupGetPageMode(data) {
+      return pdfManager.ensureCatalog('pageMode');
+    });
+
     handler.on('GetAttachments',
       function wphSetupGetAttachments(data) {
         return pdfManager.ensureCatalog('attachments');