Added file_url() function to Twig

Summary:

Create of URLs for assets within Twig Templates via a new function file_url() which maps to the drupal PHP function file_create_url() and hook_file_url_alter().

New Twig

{{ file_url('core/assets/vendor/jquery/jquery.js') }}

Equivalent PHP

echo file_create_url('core/assets/vendor/jquery/jquery.js');