XFA - Add a parser for XFA files

- the parser is base on a class extending XMLParserBase
 - it handle xml namespaces:
   * each namespace is assocated with a builder
   * builder builds nodes belonging to the namespace
   * when a node is inserted in the parent namespace compatibility is checked (if required)
 - to avoid name collision between xml names and object properties, use Symbol.

src/core/xfa/utils.js

@@ -0,0 +1,47 @@
+/* Copyright 2021 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.
+ */
+
+function getInteger({ data, defaultValue, validate }) {
+  if (!data) {
+    return defaultValue;
+  }
+  data = data.trim();
+  const n = parseInt(data, 10);
+  if (!isNaN(n) && validate(n)) {
+    return n;
+  }
+  return defaultValue;
+}
+
+function getKeyword({ data, defaultValue, validate }) {
+  if (!data) {
+    return defaultValue;
+  }
+  data = data.trim();
+  if (validate(data)) {
+    return data;
+  }
+  return defaultValue;
+}
+
+function getStringOption(data, options) {
+  return getKeyword({
+    data,
+    defaultValue: options[0],
+    validate: k => options.includes(k),
+  });
+}
+
+export { getInteger, getKeyword, getStringOption };