chore: update tsconfigs

[Rich-Harris]

extracting the test tsconfig.json changes out of #16593 so I can put up an alternative PR without duplicating all that noise

[pkg-svelte-dev]

Install the latest version of @sveltejs/kit from ec2c0d1:

pnpm add https://pkg.svelte.dev/@sveltejs/kit/c/ec2c0d123ae9558e5a97f0c2af46239ca30b6190

Open in pkg.svelte.dev: https://pkg.svelte.dev/repos/kit/pr/16644

[changeset-bot]

⚠️ No Changeset found

Latest commit: ec2c0d1

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-16644-svelte.vercel.app/

[Rich-Harris]

uncontroversial, so self-merging to unblock