guard<T> static method

Transforms a Future that may fail into something that is safe to read.

This is useful to avoid having to do a tedious try/catch. Instead of writing:

class MyNotifier extends AsyncNotifier<MyData> {
  @override
  Future<MyData> build() => Future.value(MyData());

  Future<void> sideEffect() async {
    state = const AsyncValue.loading();
    try {
      final response = await dio.get('my_api/data');
      final data = MyData.fromJson(response);
      state = AsyncValue.data(data);
    } catch (err, stack) {
      state = AsyncValue.error(err, stack);
    }
  }
}

We can use guard to simplify it:

class MyNotifier extends AsyncNotifier<MyData> {
  @override
  Future<MyData> build() => Future.value(MyData());

  Future<void> sideEffect() async {
    state = const AsyncValue.loading();
    // does the try/catch for us like previously
    state = await AsyncValue.guard(() async {
      final response = await dio.get('my_api/data');
      return Data.fromJson(response);
    });
  }
}

An optional callback can be specified to catch errors only under a certain condition.
In the following example, we catch all exceptions beside FormatExceptions.

```dart
  AsyncValue.guard(
   () async { /* ... */ },
    // Catch all errors beside [FormatException]s.
   (err) => err is! FormatException,
  );
}