Add missing visually-hidden to work with sample Drupal markup in stories

[mike-potter]

Description

Project stories often use example markup from Drupal (menus, forms, etc). Drupal commonly uses the class visually-hidden to hide elements. This fix adds the class to the outline-element.a11y.css. This mimics what several projects have had to add themselves and should be part of the inherited outline core.

Type of change

Please delete options that are not relevant.

• [X] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] This change requires a documentation update

How Has This Been Tested?

Tested on the Baseline Demo site and also being used by IDB.

• [X] Visual Testing
• [ ] Automated Testing
• [ ] Accessibility Testing

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules