Ability to disable coloring of visited links differently

Highlighting visited links is standard and good practice overall.
However, it might be a bit confusing in web applications, where links are commonly used as buttons.

It would be nice to add a SCSS variable to disable that option in Vanilla.

The issue names no files or tests; start by locating Vanilla's visited-link styling and existing SCSS configuration variables. Confirm how the proposed option should affect visited links while preserving the default behavior, then verify that the styling can be disabled as requested.

Written by the indexing model from the issue text.