Fix type inference for extended i18n schemas

[delucis]

Description

• Closes type errors using extended i18n schema #1787
• Fixes type inference for labels when a user provides additional translation strings by extending Starlight’s i18n schema
• Previously these were statically typed based on our default schema shape. Now these should infer from the content collection type if configured.

Testing

To test this, I followed these manual steps:

1. Extended the i18n schema in docs/src/content/config.ts with an extra field:

   schema: i18nSchema({ extend: z.object({ test: z.string() }) }),

2. I added a test file at docs/src/content/i18n/lang.yml to make Astro detect the collection, and added some content:

   test: test

3. I created a test component using our overrides prop type to see if it would pick up the extended field in labels:

   ---
   import type { Props } from '@astrojs/starlight/props';
   
   Astro.props.labels.test
   //                 ^ string
   ---

[changeset-bot]

🦋 Changeset detected

Latest commit: dc90395

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 30, 2024 9:48am

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[astrobot-houston]

size-limit report 📦

Path	Size
/index.html	6.1 KB (0%)
/_astro/*.js	21.57 KB (0%)
/_astro/*.css	13.45 KB (0%)

[HiDeoo]

Amazing!

Tested this against #1777 and it does not have the same issue (no idea about the criteria before a type is no longer serializable). I also tested it as-is and with the fix from #1777 and it works perfectly in both cases. 🚀