New rule: avoid_multiple_classes_in_one_file

[solid-danylosafonov]

If some functionality was already extracted to a separate class it usually doesn't make sense keeping it the file of another class.

BAD

// parent.dart

class Parent extends StatelessWidget {
...
}

class _Child extends StatelessWidget {
...
}

GOOD

// parent.dart
import 'child.dart';

class Parent extends StatelessWidget {
...
}

// child.dart
class Child extends StatelessWidget {
...
}

There should also be a way to specify exceptions, e.g. T and State<T> should be allowed in the same file by default
GOOD

class SomeWidget extends StatefulWidget {
...
}

class _SomeWidgetState extends State<SomeWidget> {
...
}