Consider adding additional refactoring challenge for operations

[tildeee]

We could consider doing this in a loop, which I find a little more compelling and would challenge the reader to do some research about for/of. But it would require some rework to the following passages.

for (const operator of [multiply, add]) {
  const result = calculate(7, 6, operator);
  console.log('Our final result from calculate is:', result);
}

Originally posted by @anselrognlie in #8 (comment)