Create pass-through trips for Seattle

[dabreegster]

https://github.com/a-b-street/abstreet/blob/3b375078b1417f37ab4d31eb9f1a86199c33c7fa/importer/src/soundcast/trips.rs#L45

Why is there so little interstate traffic on the Seattle scenarios? Because most of it both starts and ends off map. Right now, we don't import these trips. But we can do it pretty easily, using the existing code that finds a path in the huge map and snaps it to the small map.

This will likely explode the size of scenario and prebaked files, gridlock some of the currently working maps, etc. So it'll have to be done very carefully.