Fix a couple issues with anchor styling

[m-akinc]

Pull Request

🤨 Rationale

I noticed a few differences between our anchor styling and the Figma design:

1. Visited links have a distinct color (purple)
2. Keyboard focus is indicated by an outline rather than double underline
3. Disabled links should have underlines unless underline-hidden is set
4. Links with underline-hidden should not have underlines when clicked/active (despite mouse hover)
5. In the color theme, a clicked/active link should have the same, dim, gray color regardless of whether it has the prominent appearance. (Figma does not actually provide a design for the prominent appearance under the color theme, but we can extrapolate from the other themes.)

👩‍💻 Implementation

I am addressing issues 3-5. Tech debt issue #2011 tracks items 1 and 2.

To simplify things, I rewrote the anchor styles using CSS cascade layers.

🧪 Testing

Storybook snapshots

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[m-akinc]

@mollykreis mind buddying this for me?