Added and removed lines get no special styling

[siimpragi]

I ran the demo with a fenced code block that had some lines added and removed.

```js/4-5/2-3
let multilineString = `
  this is the first line
  this line is removed
  this line is also removed
  this line is added
  this line is also added
  this is the last line
`;
```

Those lines didn't seem to differentiate from "normal" lines. Here's a GIF:

The generated markup is fine. It's just that test.css hides the browser's default styles for <ins>, and <del>.

Could the stylesheet perhaps be changed so that we give appropriate background-color-s to .highlight-line-* similarly to how eleventy-base-blog does it?

If OK, would you be open to a PR?

[zachleat]

Good idea! (I made these changes to test.css! thank you)