XFA - Convert some template properties into CSS ones (#13082)

- implement few positioning properties: position, width, height, anchor;
  - implement font element;
  - implement fill element (used by font) and its children (linear, radial, ...);
  - font property is inherited from ancestor container (see https://www.pdfa.org/wp-content/uploads/2020/07/XFA-3_3.pdf#page=43) so let CSS handles that stuff;
  - in order to reduce the number of properties to set, only set non default properties and put the default in CSS;
  - set a background to some containers to be able to see them (will be removed in a future commit).

src/core/xfa/xfa_object.js

@@ -59,6 +59,7 @@ const $setSetAttributes = Symbol();
 const $setValue = Symbol();
 const $text = Symbol();
 const $toHTML = Symbol();
+const $toStyle = Symbol();
 const $uid = Symbol("uid");
 
 const _applyPrototype = Symbol();
@@ -259,6 +260,10 @@ class XFAObject {
     return dumped;
   }
 
+  [$toStyle]() {
+    return null;
+  }
+
   [$toHTML]() {
     return null;
   }
@@ -839,6 +844,7 @@ export {
   $setValue,
   $text,
   $toHTML,
+  $toStyle,
   $uid,
   ContentObject,
   IntegerObject,