fix: Use `window.fetch` instead of unpatchable fetch in `load` functions

[Lms24]

This PR makes initial_fetch and subsequent_fetch use the patched window.fetch function instead of the unpatched and stored away native_fetch function.

This change brings one big benefit: Additional behaviour added by libraries (like Sentry but there are many others) that patch window.fetch is now also invoked in event.fetch calls within client-side universal load functions.

Additionally, this provides a way for libraries to instrument fetch in load functions without causing unintended cache misses (see https://github.com/getsentry/sentry-javascript/issues/8174)

Kit patches window.fetch for two reasons:

• to warn about directly calling window.fetch it in DEV mode (this we should leave as-is)
• to clean the cache

I'm not sure if I'm missing something but cleaning the cache should be alright in load functions as well 🤔

Getting this PR merged would de-prioritize needing a dedicated handleFetch hook on the client-side (#9530) for us (which might still be worthwhile but for other reasons).

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ref: #9530 ref: https://github.com/getsentry/sentry-javascript/issues/8174 (ref: https://github.com/Lms24/gh-js-8174)

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🦋 Changeset detected

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@sveltejs/kit	Patch

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[Lms24]

@benmccann thanks for the review and sorry for missing out on the code styling. I made an additional small change (see comment above). Lmk If I can do anything else to help getting this through :)

[teemingc]

native_fetch still exists/is used in a few other places - is that deliberate?

I think it's still required for our own patches to window.fetch and when it's used by the client to retrieve load function data. If not, it would falsely fire off those "don't use window.fetch" warnings I think.

[pzuraq]

Just wanted to say, big +1 to this PR, we could really use this in our app!