feat: provide fetch to reroute

[dummdidumm]

This provides fetch as an argument to reroute, so that you can get the same benefits as the fetch provided to load functions from it. This is important (among other things) because else you might not be able to forward cookies on the server which influence where to go to.

The respond.js diff looks beefier than it is. Basically the event object is moved on top so that it's created before the reroute, because the fetch method passed to it needs to know about event. The biggest change is about cookies, which are now first created without knowing about the trailing slash, so setting new cookies (for which this is important) will wait until after a route is known.

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