[fix] add test folder to includes

[dummdidumm]

Also add some notes to tsconfig/jsonfig
Fixes #5833

I briefly thought about making this configurable through kit.files, but didn't do so because only the tsconfig includes generation would benefit from it - you'd still need to update the package.json scripts yourself.

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create-svelte	Patch
@sveltejs/kit	Patch

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