feat(react-card): add isExternal prop

[pbortnick]

🎟️ [Asana Task]() 🔍 Preview Link

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Description

This PR adds isExternal prop to <Card />. If true, the card link will open in a new tab.

This will be used in the www trust center project.

PR Checklist 🚀

Items in this checklist may not may not apply to your PR, but please consider each item carefully.

• [ ] Add Asana and Preview links above.
• [ ] Conduct thorough self-review.
• [ ] Add or update tests as appropriate.
• [ ] Conduct reasonable cross browser testing for both compatibility and responsive behavior (We have a Sauce Labs account for this, if you don't have access, just ask!).
• [ ] Conduct reasonable accessibility review (use the WAS as a guide or an axe browser plugin until we establish more formal checks).
• [ ] Identify (in the description above) and document (add Asana tasks on this board) any technical debt that you're aware of, but are not addressing as part of this PR.

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Name	Status	Preview	Comments	Updated (UTC)
react-components	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 16, 2023 7:35pm

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: e3a0d1ba846e2ffb77bea0a28aae15f31792b2e6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

| Name | Type | | -------------------------------- | ----- | | @hashicorp/react-card | Minor | | @hashicorp/react-related-content | Patch |

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