Consider generating actual `enum`s

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:
```dart
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:
```dart
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()'.