number property
Returns an integer if the operation pushes a number, or null
Implementation
@override
int? get number {
  if (code == 0) return 0;
  if (code == ScriptOp.op1Negate) return -1;
  if (code >= ScriptOp.op1 && code <= ScriptOp.op16) {
    return code - ScriptOp.op1 + 1;
  }
  return null;
}