Allow generating enums as Dart enums

[stuartmorgan]

(See previous discussion in #1816)

I have this Java declaration in a class:

  public enum StreamingFormat {
    /** Default, if the format is either not known or not another valid format. */
    UNKNOWN,

    /** Smooth Streaming. */
    SMOOTH,

    /** MPEG-DASH (Dynamic Adaptive over HTTP). */
    DYNAMIC_ADAPTIVE,

    /** HTTP Live Streaming (HLS). */
    HTTP_LIVE
  }

The resulting generated code is a class with getters like:

  static VideoAsset$StreamingFormat? get DYNAMIC_ADAPTIVE =>
      _id_DYNAMIC_ADAPTIVE.get(
          _class, const $VideoAsset$StreamingFormat$NullableType());

Because i's a bag of statics rather than an enum, very useful features like compiler-enforced exhaustive switch statements don't work. It sounds from comments in the other issue like there may be cases where clients could need to treat the enum as a class for Java compat, but for use cases where that's not necessary it would be a better dev experience to generate Dart enums, so a config option for that could be useful.