feat(css): compile utility classes library

What does this MR do?

This adds the utility classes library to Rollup's entry points so that it outputs a compiled dist/utilities.css file ready to be used in project that don't have an SCSS preprocessor setup.

This should address a community member who noticed that GlEmptyState doesn't render properly without including utility classes and doesn't have a SCSS preprocessor in his project: !1395 (comment 345400812)

Side note: Compiling utility classes might also be interesting for us to document available classes as the CSS output makes is easier to understand what utilities actually do, whereas utility mixins aren't as self-explanatory.

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.