Allow ckeditor_stylesheets to refer to a Drupal root URL

After adding support for external stylesheets the next step is to add support for Drupal root paths, in the same way that Library definitions can have a Drupal root url for CSS/JS assets.

Stylesheets that aren't external and use a '/' as the first character can be safely assumed to be Drupal root relative (this what LibraryDiscoveryParser does).

So the supported set of stylesheets paths would look like this:

ckeditor_stylesheets
  - //www.external.com/stylesheet.css
  - /libraries/your-library/stylesheet.css
  - css/stylesheet.css

The use case for this is to include CSS assets from outside the theme directory (i.e. the libraries directory).