[enh] engines: add about variable

move meta information from comment to the about variable
so the preferences, the documentation can show these information

searx/engines/json_engine.py

@@ -1,3 +1,5 @@
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
 from collections.abc import Iterable
 from json import loads
 from urllib.parse import urlencode