Fixes type error with Astro 4.15.x

[delucis]

Description

• Closes TypeScript error in utils/i18n.ts: Missing fallbackType property #2271
• Adds handling for the new i18n.routing.fallbackType option added in Astro 4.15.0
• Tested locally with both the current version of Astro used in the monorepo and after installing the latest Astro release to ensure type checking still passes

[changeset-bot]

🦋 Changeset detected

Latest commit: dbbdfd9

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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[astrobot-houston]

size-limit report 📦

Path	Size
/index.html	6.15 KB (0%)
/_astro/*.js	22.33 KB (0%)
/_astro/*.css	13.72 KB (0%)

[HiDeoo]

Looks perfect to me 👍

Confirmed locally that this fixes the issue with the latest Astro release in the monorepo but also in a user project running tsc --noEmit like mentioned in the issue.

P.S. Noticed a few other issues in the monorepo with the latest Astro release that we would need to handle when updating the monorepo version, e.g. a type issue with getViteConfig from astro/config and also the location of .astro/types.d.ts at the root of the monorepo which seems weird.

[delucis]

P.S. Noticed a few other issues in the monorepo with the latest Astro release that we would need to handle when updating the monorepo version, e.g. a type issue with getViteConfig from astro/config and also the location of .astro/types.d.ts at the root of the monorepo which seems weird.

Yeah, I also noticed those. Asked @florian-lefebvre about the types.d.ts one, but didn’t get a chance to track down the exact version change causing that.