[fix] consider paths.assets for version.json

[baseballyama]

fix: #4188

Checklist

I checked behavior with adapter-node.

• It can fetch version.json if kit.paths.assets has custom value
• It can fetch version.json if kit.paths.assets doesn't set value

I couldn't find how to write tests.
So could you please teach me if there are some sample existing tests?

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[Rich-Harris]

Thanks — this would fix the issue (agree that it's quite hard to add a test for this, I suppose we could set paths.assets to http://localhost:${TEST_PORT}/assets and check it works, but that feels like a task for another day) but it's not 100% future-proof, because one day we'd like to introduce dynamic basepaths (#595) so that Kit apps work on IPFS/the internet archive/situations where the basepath is set by an environment variable/etc.

I think we can support that by just using paths.assets instead of paths.base (if assets is unset, it defaults to base) — 5cb8870

[baseballyama]

VITE_SVELTEKIT_APP_VERSION_FILE is super specific name, so I assigned complete path.
But I agree with you that building complete path at runtime is more flexible if we consider dynamic basepath.👍

Regarding testing, I remember that there have been some issues regarding basepath in the past.,
so if we have a testing mechanism, it would be great.