Align brand color names in Figma and CSS variables with our brand guidelines

[valerybugakov]

Based on this discussion.

The logo's purpleviolet #A112FF is officially called "Sourcegraph Vivid Violet", while there is also another primary color for action buttons called "Sourcegraph Blurple" #5033E1. Having the vivid violet color be named "purple" could be easy to confuse with Sourcegraph blurple, so I'd suggest renaming this to e.g. --logo-violet, --brand-vivid-violet or --sg-vivid-violet (violet instead of purple).

[github-actions]

Heads up @AlicjaSuska @umpox @valerybugakov @5h1rU @pdubroy - the "team/frontend-platform" label was applied to this issue.