Add code diff highlighting

[thgaskell]

Adds syntax highlighting for code diffs in markdown:

Light

Dark

Markdown

```jsx
function HighlightSomeText(highlight) {
  if (highlight) {
    // diff-remove
    console.log('Highlighted text found');
    // diff-add
    console.log('Highlighted text found');
    return 'This text is highlighted!';
  }

  return 'Nothing highlighted';
}
```

[jaygiang]

LGTM ✅