playground.wordpress.net ships an optimized WordPress build where most
static assets are removed to keep the initial download snappy. However,
some backend code actually depends on those assets being present as seen
in #729 and #770. This PR allowlists additional files:

* wp-includes/images/media
* wp-includes/fonts/dashicons.svg
* wp-content/plugins/akismet/_inc

To test, run Playground locally, go to this URL, and confirm it displays
the list of files as shown below.

http://localhost:5400/website-server/#%7B%22landingPage%22:%22/test.php%22,%22steps%22:[%7B%22step%22:%22writeFile%22,%22path%22:%22/wordpress/test.php%22,%22data%22:%22%3Cpre%3E%3C?php%20var_dump(glob(%27/wordpress/wp-includes/fonts/*%27));%20var_dump(glob(%27/wordpress/wp-includes/images/media/*%27));%20var_dump(glob(%27/wordpress/wp-content/plugins/akismet/_inc/*%27));%22}]}

```
array(3) {
  [0]=>
  string(60) "/wordpress/wp-includes/fonts/class-wp-font-face-resolver.php"
  [1]=>
  string(51) "/wordpress/wp-includes/fonts/class-wp-font-face.php"
  [2]=>
  string(42) "/wordpress/wp-includes/fonts/dashicons.svg"
}
array(9) {
  [0]=>
  string(47) "/wordpress/wp-includes/images/media/archive.png"
  [1]=>
  string(45) "/wordpress/wp-includes/images/media/audio.png"
  [2]=>
  string(44) "/wordpress/wp-includes/images/media/code.png"
  [3]=>
  string(47) "/wordpress/wp-includes/images/media/default.png"
  [4]=>
  string(48) "/wordpress/wp-includes/images/media/document.png"
  [5]=>
  string(51) "/wordpress/wp-includes/images/media/interactive.png"
  [6]=>
  string(51) "/wordpress/wp-includes/images/media/spreadsheet.png"
  [7]=>
  string(44) "/wordpress/wp-includes/images/media/text.png"
  [8]=>
  string(45) "/wordpress/wp-includes/images/media/video.png"
}
array(3) {
  [0]=>
  string(59) "/wordpress/wp-content/plugins/akismet/_inc/akismet-admin.js"
  [1]=>
  string(62) "/wordpress/wp-content/plugins/akismet/_inc/akismet-frontend.js"
  [2]=>
  string(53) "/wordpress/wp-content/plugins/akismet/_inc/akismet.js"
}
```