Define sparkline color variants

The sparkline-chart component was built to support different color variants for the hover-markpoints:

For the first iteration, it was decided that the only color that was going to be added is #000.

Going forward, we need to decide on what other colors should be supported, or if black is enough, to remove the variant-prop from the component.

Relevant Issues / Discussions:

• Sparkline Chart Issue: gitlab-org/gitlab-ui#366 (closed)
• Discussion about the color variants: gitlab-org/gitlab-ui#366 (comment 228943572)
• MR that adds the sparkline component: gitlab-org/gitlab-ui!834 (merged)