feat: allow setting gcds-nav-links as external

[radmorecameron]

📝 Summary | Résumé

This PR allows setting gcds-nav-links as external (it will open in a new tab and shows the same icon, similar to how the gcds-link functions)

This change is motivated by the need that people might need to navigate to a help/instructions page while in a wizard and wouldn't want to lose their progress.

🧩 Related Issues | Cartes liées

• Closes: feat: ability to have nav-links open in a new tab #1256

🧪 Test instructions | Instructions pour tester la modification

1. checkout this branch
2. build web npm run build:web
3. build storybook npm run build-storybook
4. run storybook npm run start-storybook
5. go to nav-link storybook
6. look at external example

✍️ Author checklist | Liste de vérification de l'auteur

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• This PR introduces a minor change (use feat:)

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• This PR does not break existing functionality. I have completely tested the functionality of these changes.
• This PR does not introduce any changes to component names, properties, values, or behaviour.

Ready for review: (all items must be checked)

• I have tested the English and French versions of the changes, and can verify that all content is accurate and properly displayed in both languages.
• I have tested these changes on mobile viewports.
• I have tested these changes across multiple supported browsers.
• I have checked accessiblity and ensured all accessiblity tests pass.
• I have added tests for added functionality or changed existing tests, as needed.
• Visual or content changes remain aligned with design tokens and component API standards.
• Test instructions are clear and reproducible.

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⚠️ Impact/Risks | Risques

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[ethanWallace]

This is great! Thank you for the contribution! We will forward this on to our design team to see if we can get this reflected in our design files before merging/releasing.

• I have tested the changes and functionality using the test instructions.
• I have confirmed test coverage is adequate.
• I have reviewed the code for clarity, maintainability and potential issues.

[daine]

Thanks @radmorecameron for this PR! The rest of the team is taking a look at it this sprint, we'll be needing our design team to take a look at it as well and provide their approval. Appreciate your work and your patience!

[daine]

Hi @radmorecameron although the code looks good, we're considering potentially other approaches and solutions for the original feature request. Someone from our team will reach out to you in the linked issue #1256 . Thank you for the contribution, and we appreciate your patience!