JS -- Implement doc object

* https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/js_api_reference.pdf#page=335
 * it has all the properties/methods defined in the spec
 * unimplemented methods are there but with an empty body to avoid exception when calling an undefined method
 * implement zoom, zoomType, layout, pageNum, ...

src/scripting_api/constants.js

@@ -0,0 +1,26 @@
+/* Copyright 2020 Mozilla Foundation
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+const ZoomType = Object.freeze({
+  none: "NoVary",
+  fitP: "FitPage",
+  fitW: "FitWidth",
+  fitH: "FitHeight",
+  fitV: "FitVisibleWidth",
+  pref: "Preferred",
+  refW: "ReflowWidth",
+});
+
+export { ZoomType };