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.

web/xfa_layer_builder.css

@@ -220,6 +220,11 @@
   text-align: center;
 }
 
+.xfaLink {
+  width: 100%;
+  height: 100%;
+}
+
 .xfaCheckbox,
 .xfaRadio {
   width: 100%;