ckeditor_stylesheets can now refer to a Drupal root URL

After adding support for external stylesheets the library definitions can have a Drupal root URL for CSS/JS assets.

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

Before

# your_theme.info.yml

ckeditor_stylesheets
  - //www.external.com/stylesheet.css
  - css/stylesheet.css

After

# your_theme.info.yml

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).