Add color util classes for backgrounds and text

What does this MR do?

• util classes for .bg-variant-value and .text-variant-value for colors.
  variant == 'primary', 'secondary', 'success', 'warning', 'danger'
  value == 50, 100, 200, ... 950 from Colors

• .text-1 to .text-6 for the sizes in our Type Scale

Example output (kept color vars for clarity):

.bg-success-100 {
  background-color: $green-100;
}

.text-warning-950 {
  color: $orange-950;
}

.text-4 {
  font-size: 20px;
}

What are the relevant issue numbers?

Needed for gitlab-ui!202 (merged)

Does this MR meet the acceptance criteria?

• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides