Use shared node_class decorator

Author: Cito

src/graphql/language/__init__.py

@@ -34,6 +34,7 @@
     Location,
     Token,
     Node,
+    node_class,
     # Each kind of AST node
     NameNode,
     DocumentNode,
@@ -207,6 +208,7 @@
     "is_type_system_definition_node",
     "is_type_system_extension_node",
     "is_value_node",
+    "node_class",
     "parse",
     "parse_const_value",
     "parse_type",