feat(css): add new stateful utility mixins

What does this MR do?

Adds some new stateful utility mixins:

hover-gl-cursor-pointer
hover-gl-border-b-solid
hover-gl-border-blue-200
hover-gl-bg-blue-50

One of these did not have an existing mixin, so this MR also adds the base utility mixin:

gl-border-blue-200

This will allow us to remove these class definitions from gitlab-org/gitlab and resolve gitlab#217994 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• [n/a] If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• [n/a] If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• [n/a] 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:
  • [n/a] GitLab: mr_url
  • [n/a] Customers Portal: mr_url
  • [n/a] Status Page: mr_url
• [n/a] Added the ~"component:*" label(s) if applicable.