[api-minor] Add support for URLs in the document outline

Re: issue 5089.
(Note that since there are other outline features that we currently don't support, e.g. bold/italic text and custom colours, I thus think we can keep the referenced issue open.)

src/display/api.js

@@ -714,6 +714,7 @@ var PDFDocumentProxy = (function PDFDocumentProxyClosure() {
      *   italic: boolean,
      *   color: rgb array,
      *   dest: dest obj,
+     *   url: string,
      *   items: array of more items like this
      *  },
      *  ...