chore: deprecate pathless cookie, warn on path: ''

[Rich-Harris]

This prints a warning if you call cookies.set(...) or cookies.delete(...) without specifying a path, as part of #9299.

I think we should change the behaviour of path: '' in SvelteKit 2.0 as well. Currently, it's equivalent to leaving it unspecified. It would be more useful (and more consistent with other places where paths are resolved) if it meant 'the current path' instead — the fact that it doesn't is a quirk of how cookie headers are serialized, rather than something deliberate. As such, I've also added a warning that it will change. (I expect very few people are doing path: '' today.)

In development, the warning also points to the callsite, making it easy to fix.

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