lint [nfc]: Suppress constant_identifier_names in generated files

This prevents us from making private `JsonEnum` classes
as the generated `EnumMap` does not follow the rule of
being a lowerCamelCase identifier.

For example, `_Foo` generates `_$_FooEnumMap` which
triggers an analyzer complaint.

Author: sirpengi

build.yaml

@@ -4,5 +4,6 @@ targets:
       source_gen:combining_builder:
         options:
           ignore_for_file:
+            - constant_identifier_names
             # https://github.com/google/json_serializable.dart/issues/625
             - unnecessary_cast