Allow overloads with `async` vs `sync` lambda parameters

[BSteffaniak]

Similar to #430

public sync forEach(func(Type, Int, List<Type>)) => this {
  var Int i = 0

  for (value in this) {
    func(value, i++, this)
  }
}

public async forEach(async func(Type, Int, List<Type>)) => this {
  var Int i = 0

  for (value in this) {
    func(value, i++, this)
  }
}

where it calls the respective one depending on if the lambda passed to it is async or not