A couple of (small) tweaks of the AnnotationStorage (PR 12173 follow-up)

- Initialize the `AnnotationStorage`-instance, on `PDFDocumentProxy`, lazily.
 - Change the `AnnotationStorage` to use a `Map` internally, rather than a regular Object (simplifies the following points).
 - Let `AnnotationStorage.getAll` return `null` when there's no data stored, to avoid unnecessary parsing on the worker-thread. This ought to "just work", since the worker-thread code *should* already handle the `!annotationStorage` case everywhere.
 - Add a new `AnnotationStorage.size` getter, to be able to easily tell if there's any data stored.

src/display/api.js

@@ -632,14 +632,13 @@ class PDFDocumentProxy {
   constructor(pdfInfo, transport) {
     this._pdfInfo = pdfInfo;
     this._transport = transport;
-    this._annotationStorage = new AnnotationStorage();
   }
 
   /**
    * @type {AnnotationStorage} Storage for annotation data in forms.
    */
   get annotationStorage() {
-    return this._annotationStorage;
+    return shadow(this, "annotationStorage", new AnnotationStorage());
   }
 
   /**