fix: use window.fetch for server load fetch requests

[Lms24]

This PR is a follow up on #10009 which ensured that the fetch function from the load event would call window.fetch so that libraries (like Sentry but there are many others) can patch it. Unfortunately, I missed back then that there's another kind of fetch request made by SvelteKit: Whenever a user navigates to a route that contains a server-load function, a request is made to fetch the server-load data.

This PR ensures that this server-load data request is also patchable by basically using the same mechanism introduced in #10009. I also added a test that would prior to this change fail.

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🦋 Changeset detected

Latest commit: 813f05f

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@sveltejs/kit	Patch

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[Rich-Harris]

preview: https://svelte-dev-git-preview-kit-13315-svelte.vercel.app/

this is an automated message

[dummdidumm]

thank you!

[Lms24]

Thanks for the quick turn around!