lib: implement stub for word search

lib/models/common/jlpt_level.dart

@@ -0,0 +1 @@
+enum JlptLevel { none, n5, n4, n3, n2, n1 }

lib/models/jmdict/word_search_result.dart

@@ -1,3 +1,5 @@
 class WordSearchResult {
   // TODO: implement me
+
+  Map<String, dynamic> toJson() => {};
 }