[fix] add types properly in exports in package.json

[dummdidumm]

Fixes #2863
Also removes the somewhat weird types export "./types" which is redundant: you get these through the default import, too. This shouldn't be a breaking change either (at least right now), as only TypeScript would be able to deal with that import as it's type-only, and TypeScript isn't officially supporting the exports field yet. I don't know who would do import { SomeInterface } from '@sveltejs/kit/types'; anyway if you can do import { SomeInterface } from '@sveltejs/kit/types';.

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