WIP: feat: Add focus-visible polyfill

What does this MR do?

This adds the focus-visible polyfill to allow for elements that need to have an outline when selected via a keyboard (e.g. tabbing to an element) but hide said outline when a mouse event happens.

This is only supported via firefox, so this polyfill is meant to fill in the gap for the rest of the supported browsers

Web Incubator upstream repository: https://github.com/WICG/focus-visible

Can I Use? Link: https://caniuse.com/#feat=css-focus-visible

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.