fix: page response missing CSP and Link headers when return promise in `load`

[0x221A]

closes #11801

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Latest commit: c8eb6825bdab3302bf739c7ce46d2a57305aefd1

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[darg-2036]

Would love to see this merged

[teemingc]

Thank you for the PR. I think it's good to merge, but is it possible if we add some kind of test such as one that checks that the CSP headers are included in the server response? Maybe as an additional test in this test suite: https://github.com/sveltejs/kit/blob/6f273fb8a95429c0ff544d4ccd8741c7e1f4ad18/packages/kit/test/apps/options/test/test.js#L115-L139

[0x221A]

@eltigerchino sure!

[teemingc]

Thank you so much!