Re-factor Catalog._collectJavaScript to use a Map rather than an Object

Given that this only an internal helper method, used by the `Catalog.{javaScript, jsActions}` getters, this change simplifies iteration of the returned data.
We can also (slightly) re-factor the code of the `jsActions` getter, and remove an obsolete[1] JSDoc-comment from the `openAction` getter.

---
[1] Not really relevant now that we've got proper scripting support.

src/core/document.js

@@ -517,7 +517,6 @@ class Page {
       this.pageDict,
       PageActionEventType
     );
-
     return shadow(this, "jsActions", actions);
   }
 }