XFA - Add a layer to display XFA forms (#13069)

- add an option to enable XFA rendering if any;
  - for now, let the canvas layer: it could be useful to implement XFAF forms (embedded pdf in xml stream for the background and xfa form for the foreground);
  - ui elements in template DOM are pretty close to their html counterpart so we generate a fake html DOM from template one:
    - it makes easier to translate template properties to html ones;
    - it makes faster the creation of the html element in the main thread.

src/core/xfa/utils.js

@@ -74,7 +74,7 @@ function getMeasurement(str, def = "0") {
   }
   return {
     value: sign === "-" ? -value : value,
-    unit,
+    unit: unit || "pt",
   };
 }