Remove uninit value

[jhnaldo]

Instead of using the uninit value for not-yet-initialized fields for records, we initialize fields with undefined as described Object Internal Methods and Internal Slots in ECMA-262:

Unless specified otherwise, the initial value of an internal slot is the value undefined.

While it describes the initial value only for internal slots of objects, we apply this interpretation to fields of any records. Therefore, this PR updates the contains check for field types as follows:

 def contains(v: Option[Value], heap: Heap): Boolean = v match
    case Some(Undef)        => true // Undef represents uninitialized value
    case Some(value: Value) => this.value.contains(value, heap)
    case None               => this.absent
}