Use understrap_get_screen_reader_class() for a.skip-link

[IanDelMar]

Description

This PR adds understrap_get_screen_reader_class() and uses that function to readd Bootstrap's screen reader text classes to a.skip-link.

Motivation and Context

Adding focusable screen reader text styles to .skip-link and removing .sr-only and .sr-only-focusable from a.skip link permanently unhides a.skip link for child themes not loading Understrap's theme.css and not including a recent version of assets/understrap/theme/_theme.scss in child-theme.scss. Removing the classes has been a breaking change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (a code change that neither fixes a bug nor adds a feature)
• Styling (changes that do not affect the meaning of the code (white-space, formatting, missing semi-colons, etc))
• Development (changes that affect the build system or external dependencies)
• Documentation (changes that affect existing inline documentation)

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• (Optional) My change requires a change to the documentation.
• (Optional) I have updated the documentation accordingly.
• (Optional) My change requires a change to the translations.
• (Optional) I have updated the translations accordingly.
• composer phpcs has passed locally.
• composer php-lint has passed locally.
• composer phpmd has passed locally.
• composer phpstan has passed locally.
• I have read the CONTRIBUTING document.

Related Issues or Roadmap requests

Fixes #2036

Further comments

@bacoords I suggest to add this to v1.2.2.