feat: export CookieSerializeOptions type

[phdoshi]

Creates and exports a custom CustomCookieSerializeOptions type for issue #11799, edited in the following files:

• types.d.ts
• cookie.js
• public.d.ts
• index.d.ts

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[benmccann]

I'm not an expert at the types setup, but there's nothing else in this file importing from ../runtime which makes me think it's the wrong place to put it. everything else is being imported from private

Also, let's drop Custom from the name. All of the types we defined are specific to SvelteKit. No need to prefix all of them with Custom...