feat: Upgrade page caching

[elliott-with-the-longest-name-on-github]

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This completes #4549. A couple of notes for consideration before merging:

The cache.private option will override (perhaps confusingly?) behavior that's currently in place -- for example, cache.private === false will cause the cache-control header to be public, even if a credentialed fetch or the session object is accessed (whether in init or in load). Because of this, I was extra specific in the tests, asserting that the override worked in every case. Even if this is a bit confusing at first, I can't think that going half way (having it not override the defaults) in some cases would be any clearer.

[changeset-bot]

🦋 Changeset detected

Latest commit: fa7a5ce

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

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@sveltejs/kit	Patch

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[elliott-with-the-longest-name-on-github]

Note: Cannot replicate test failure locally. ☹️

[Rich-Harris]

thank you, this is a great PR!