fix: minor type issues in starlight

[jorenbroekema]

Description

• Closes # N/A didn't create an issue since the PR is so tiny
• What does this PR change? Fixes some types
• Did you change something visual? No

Here's the type issues I was getting, skipLibCheck unfortunately doesn't work for TS files (only .d.ts) so this is a real blocker when trying to lint/compile my project which uses starlight.

[changeset-bot]

🦋 Changeset detected

Latest commit: d085951

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

This PR includes changesets to release 1 package

Name	Type
@astrojs/starlight	Patch

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Name	Status	Preview	Updated (UTC)
starlight	✅ Ready (Inspect)	Visit Preview	Apr 9, 2024 11:05am

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[HiDeoo]

Thanks for your contribution, super appreciated! 🙌

Left a few comments/questions inline to make sure we properly understand the issues and fix them properly but this is clearly something that needs to be fixed. Thanks for opening up a PR.

[delucis]

Thank you for these fixes and your responsiveness making changes @jorenbroekema! Appreciate the contribution 🌟