Ensure props are loaded from matching endpoint during client-side navigation

[Rich-Harris]

Fixes #4038.

Credit to @aolose for the first version of this (#4139). I tried to work on that branch but things had diverged too much, and I was concerned that the implementation came with fair bit of intrinsic complexity. This implementation is slightly less sophisticated — if an endpoint falls through during client-side navigation, Kit won't find which endpoint actually matches and allow the client to short-circuit the next attempt — but it's less code and easier to follow. Fallthroughs are something of an edge case so I'm okay with them not being fully optimised.

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