docs(showcase): add itihon.github.io/isomorphic-validation to showcase

[itihon]

Adds https://itihon.github.io/isomorphic-validation/ per https://github.com/withastro/starlight/blob/main/CONTRIBUTING.md#showcase.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 99c25bd

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[netlify]

✅ Deploy Preview for astro-starlight ready!

Name	Link
🔨 Latest commit	99c25bd
🔍 Latest deploy log	https://app.netlify.com/sites/astro-starlight/deploys/67a096f2b9aca100083679a6
😎 Deploy Preview	https://deploy-preview-2842--astro-starlight.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 100 (no change from production) Accessibility: 100 (no change from production) Best Practices: 100 (no change from production) SEO: 100 (no change from production) PWA: - View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify site configuration.

[astrobot-houston]

Hello! Thank you for opening your first PR to Starlight! ✨

Here’s what will happen next:

1. Our GitHub bots will run to check your changes.
   If they spot any issues you will see some error messages on this PR.
   Don’t hesitate to ask any questions if you’re not sure what these mean!

2. In a few minutes, you’ll be able to see a preview of your changes on Netlify 🤩

3. One or more of our maintainers will take a look and may ask you to make changes.
   We try to be responsive, but don’t worry if this takes a few days.

[trueberryless]

Thank you very much for your first contribution to the Starlight project! The changes you made look good to me, thanks for the addition! 🎉

[HiDeoo]

Amazing new addition to the showcase 🌟

Thanks for your contribution 🙌