[api-minor] Move the page reference/number caching into the API

Rather than having to handle this *manually* throughout the viewer, this functionality can instead be moved into the API which simplifies the code slightly.

src/core/worker.js

@@ -419,6 +419,7 @@ class WorkerMessageHandler {
           return {
             rotate,
             ref,
+            refStr: ref?.toString() ?? null,
             userUnit,
             view,
           };