Functional type to customize the field naming and access.

import scala.collection.immutable.Set
import io.github.scalats.ast.TypeRef
import io.github.scalats.core.{
  Settings, Field, FieldMapper
}

class CustomFieldMapper extends FieldMapper {
  def apply(
    settings: Settings,
    ownerType: String,
    propertyName: String,
    propertyType: TypeRef) =
    Field("_" + propertyName, Set.empty)
}