TypedReducer is not extendable

Current TypedReducer looks:

```dart
class TypedReducer implements ReducerClass {
/// A [Reducer] function that only accepts an action of a specific type
final State Function(State state, Action action) reducer;

/// Creates a reducer that will only be executed if the dispatched action
/// matches the [Action] type.
TypedReducer(this.reducer);

@override
State call(State state, dynamic action) {
if (action is Action) {
return reducer(state, action);
}

return state;
}
}
```

I prefer to extend TypedReducer instead pass function. It looks more convenient and by this reason I am using next version of TypedReducer:

```dart
abstract class TypedReducer implements ReducerClass {
factory TypedReducer(State Function(State state, Action action) reducer) =>
_TypedReducer(reducer);

TypedReducer.default();

@override
State call(State state, dynamic action) {
if (action is Action) {
return reduce(state, action);
}

return state;
}

State reduce(State state, Action action);
}

class _TypedReducer extends TypedReducer {
final State Function(State state, Action action) reducer;

_TypedReducer(this.reducer) : super.default();

@override
State reduce(State state, Action action) => reducer(state, action);
}

```
It doesn't make any conflict with current code and I think this version can be helpful for other users of this library.