No mapper is generated for stanadlone enum files that aren't part of any class

[agilob]

No mapper is generated for an enum when the enum isn't used in any class and has its own file, but has fields annotated with @JsonValue. Since @JsonSerializable is allowed only on classes, not enums, there's no way to deserialize such enums.

Enum also has part x.g.dart, but it's never generated.

[kevmoo]

Duplicate of #778