fix: redirect trailing slash normalization relatively instead of against root

[HoldYourWaffle]

Fixes #13718.
As mentioned there: Location is allowed to be relative, so I just... did that.
For the scenario listed in the issue that means /sveltekit/sverdle/ is redirected with Location: ../sverdle, which correctly resolves to /sveltekit/sverdle.

Remaining things to consider

• 1. Still need to inline the get-relative-path code that does the heavy lifting, but I'd like to confirm this is the correct solution first.
• 2. What should tests for this look like? Should I create a whole adapter-node+dynamic base project or keep it simple and just check if the path is relative?
• 3. adapter-node currently imports the internal relative_pathname utility through @sveltejs/kit/node, which feels a bit weird, is there a better place for this?
• 4. I'm not 100% confident this doesn't regress #2515, unfortunately there's no minimal reproduction and #4414 didn't add a test. I tried various combinations of slashes and protocol-ish paths, but even taking out #4414's fix I always got 404 rather than redirected. Is this perhaps taken care of elsewhere now? Perhaps @wbster or @benmccann can clarify.

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