Consider generating actual `enum`s

[Levi-Lesches]

A Protobuf enum gets generated to a class that extends ProtobufEnum, to ensure they provide name and value. But now with enhanced enums, they can be actual enums and have better switch support, namely "switch expressions", which doesn't allow exhaustiveness with the current implementation.

Currently, the ProtobufEnum class has two final fields that need to be passed to a constructor. Enhanced enums can't extend classes, but they can implement them. So enums can be generated as:

enum MyEnum implements ProtobufEnum {
  a("a", 0), 
  b("b", 1),
  c("c", 2);

  @override final String name;
  @override final int value;
  const MyEnum(this.name, this.value);
}

Which will then enable proper exhaustiveness checking in switch expressions, like the following:

String getName(MyEnum e) => switch (e) {
  MyEnum.a => "a",
  // Error: The type 'MyEnum' is not exhaustively matched by the switch cases since it doesn't match 'MyEnum.b'.
}

Currently, the only warning is just

The type 'MyEnum' is not exhaustively matched by the switch cases since it doesn't match 'MyEnum()'.