feat: Add nav variant to Link

[suzubara]

We have an existing Link component that should be adjusted to handle a new variant prop called nav.

Requirements:

• Add nav as another possible value to the StyledLinkProps variant type definition
• Adjust the logic in the linkClasses function so that it includes the usa-nav__link class when the variant prop is nav
• Add a storybook example to Link.stories.tsx for <Link variant="nav">

[haworku]

This could be accomplished by adding a navLink variant to the new <Link /> #309 Renamed the issue to reflect that.

[SirenaBorracha]

@suzubara or @haworku could you please review and let me know if anything looks off? Otherwise I'll put up a PR. My branch: https://github.com/trussworks/react-uswds/tree/ak-feat-add-nav-variant-to-link-166

[haworku]

@SirenaBorracha hello, new contributor 😄

The ternary looks close but I think there's a change we still need. It is easier to discuss if we can comment on the code diff directly in a PR rather than here in an issue away from the code. Go ahead and open a PR from your branch. You can make a "draft" PR and/or you can put the yellow "wip" work in progress label on it.