Add an option for `jsonSerializable` annotation to choose what `enum` property `name` or `index` to use for serialization.

Now [generated code](https://github.com/google/json_serializable.dart/issues/1286) contains such statement for `ThemeMode` `enum`:
```dart
const _$ThemeModeEnumMap = {
ThemeMode.system: 'system',
ThemeMode.light: 'light',
ThemeMode.dark: 'dark',
};
```

So `enum` values are serialized as `String`s but having a possibility to serialize `enum` values as `int`egers would be a good option.

```dart
const _$ThemeModeEnumMap = {
ThemeMode.system: 0,
ThemeMode.light: 1,
ThemeMode.dark: 2,
};
```