Populate event.params before calling handle

[Rich-Harris]

This is made possible by #4330 — now that we can match routes without needing to load/run them, we can populate event.params before calling handle, instead of mutating the event object as we work our way through the route manifest.

A nice follow-up would be to expose the route key, making it possible to address #3840 without introducing a new hook.

Closes #1194.

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