Add createBackFlow

[Olian04]

const A = createQueryAtom({
  key: 'value',
  default: 0,
});

const B = createExternalSelector({
  default: 0,
});

createBackFlow({
  from: B,
  to: A,
});

Mock implementation:

const createBackFlow = <T>(props: {
  from: ImplementsKey & ImplementsSubscribe & ImplementsGet<T>,
  to: ImplementsKey & ImplementsSet<T>, 
}) => createEffect(() => {
  props.to.set(props.from.get());
});

Real implementation should also do flow analysis to prevent cyclical dependencies (infinite loops).