chore: Add utility classes library 1st iteration

I worked on generating a group of utility classes that cover (probably not fully) the following areas:

• spacing based on gitlab’s spacing scale.
• colors based on gitlab’s colors and themes.
• borders
• font weights
• font size
• font family
• line height

Conventions try to stick strictly with gitlab-org/frontend/rfcs#4 (closed) and are also heavily inspired in the tailwind CSS utility-first framework. I decided to cover those areas only because I need them to style tabs !623 (merged)