add back focus box shadow to `btn-link`

Created by: RyanBerliner

5.2.0-beta1 looks to have accidentally dropped the box shadow around btn-link buttons on focus. You can see this by tab navigating to a btn-link on both 5.1.x and 5.2.0-beta1 and comparing

• 5.2 beta button example
• 5.1 button example

This is because the --#{$prefix}btn-focus-shadow-rgb variable is undefined in the context of btn-link. My solution here does involve code duplication from the button-variant() mixin - I'm certainly open to other solutions, however this illustrates the problem and a potential solution.

Preview link: https://deploy-preview-36366--twbs-bootstrap.netlify.app/docs/5.2/components/buttons/#examples