fix(GlLabel): Add focus state

What does this MR do?

A tiny MVC for #803 (closed)

Does this MR meet the acceptance criteria?

Screenshots

Issues

This merge introduces two pieces of ~"technical debt" in the form of !important

@mixin gl-label-focus() {
  box-shadow: $focus-ring, var(--label-inset-border) !important;
}

box-shadow: var(--label-inset-border) !important;

These are needed until we can remove some application specific code in the GitLab project which sets the inset to the currentColor.

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.