XFA - Add <a> element in button when an url is detected (bug 1716758)

- it aims to fix https://bugzilla.mozilla.org/show_bug.cgi?id=1716758;
  - some buttons have a JS action with the pattern `app.launchURL(...)` (or similar) so extract when it's possible the url and generate a <a> element with the href equals to the found url;
  - pdf.js already had some code to handle that so this patch slightly refactor that.

test/unit/annotation_spec.js

@@ -1108,7 +1108,7 @@ describe("annotation", function () {
           jsEntry: "window.open('http://www.example.com/test.pdf')",
           expectedUrl: new URL("http://www.example.com/test.pdf").href,
           expectedUnsafeUrl: "http://www.example.com/test.pdf",
-          expectedNewWindow: undefined,
+          expectedNewWindow: false,
         });
 
         // Check that we accept a white-listed {Stream} 'JS' entry.