Inconsistent support for "alt" link styling #4582

Describe the bug

USWDS includes styles for an "alt" link modifier, where "alt" appears to be for a link shown on a dark background.

However, these styles are inconsistent, because:

• It only applies to external links
• It only applies reverse link styling to the external link icon, not the text itself

Steps to reproduce the bug

1. Go to https://designsystem.digital.gov/components/link/
2. Right-click and "Inspect element" on one of the preview links
3. Add the class "usa-link--alt"

Expected: Reverse styles are shown, regardless if the link is external.
Actual: Nothing happens.

System setup

• USWDS version: v2.13.2

Additional context

There may be multiple ways to solve this:

1. Add support for "alt" links consistently as applying reverse link styling
   • Action: Add .usa-link--alt styles that apply to all links, not just external links
   • Action: If needed, improve .usa-link--external.usa-link--alt styles to also apply styling to the link text, not just the icon.
     • Technically, the icon styles are redundant with new improved external link icon styling from Links: Use add-color-icon mixin with currentColor for external link #4297, and could be removed.
2. Remove support for "alt" links and only support reverse link styling within usa-dark-background
   • Action: Remove .usa-link--alt styles
     • Technically, they are already redundant with new improved external link icon styling from Links: Use add-color-icon mixin with currentColor for external link #4297
   • Consider: usa-dark-background is opinionated about specific background color (base-dark)