error trying to use string-extra functions

In order to access the Symfony\Component\String\UnicodeString class in twig, you need to add it as an allowed class in your settings.php with the twig_sandbox_allowed_classes setting. For example:

$settings['twig_sandbox_allowed_classes'] = [
  'Symfony\Component\String\UnicodeString',
  'Drupal\Core\Template\Attribute',
];

See more on how this is processed in the TwigSandboxPolicy.

This should be documented on both the module page and the README