[chore] bump TS to 4.8

[dummdidumm]

Fixes a breaking change introduced by TS: NonNullable is no longer never when strict null checks are false: microsoft/TypeScript#50519

Closes #6387
This isn't a breaking change because everything is backwards compatible and only new users of create-svelte get a higher version (but it works with the older one, too)

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[changeset-bot]

🦋 Changeset detected

Latest commit: fb7b676

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

This PR includes changesets to release 2 packages

Name	Type
create-svelte	Patch
@sveltejs/kit	Patch

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[benmccann]

thanks for figuring this out!!