[fix] harmonize cookie path and add dev time warnings

[dummdidumm]

• Warnings will be logged to the console if it's detected that there's likely misusage of the path parameter.
• More documentation
• Fix mismatch of cookie path between /foo and /foo/__data.json server-load calls by emulating browser behavior of the default path

Fixes #6609

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🦋 Changeset detected

Latest commit: 7567c53

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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