[Feature Request] Add parent field to RefinementCtx

[Faithfinder]

I'm looking to transform an object like this:

const Raw = {
  value1: 1,
  value1_prev: 2,
  value2: 3,
  value2_prev: 4,
}

into an object like this:

const Parsed = {
  value1: {
    current: 1,
    prev: 2,
  },
  value2: {
    current: 3,
    prev: 4,
  }
}

It would be nice to define it via value1Schema.transform(), instead of a giant transform on the parent object, however I would need access to the parent object on the field to achieve that. Is that possible?