Network: Connect Footway<>Centerline to Generated Sidewalk

Example:
* https://a-b-street.github.io/speedwalk/#17.66/52.763668/13.246949
* https://www.mapillary.com/app/?pKey=1623087731621290&focus=photo&lat=52.7641425&lng=13.2451492&z=17

I just added the crossing=informal to signal that the only way to use this footway is to just cross the road and cross the grass-verge.

I did not add the separate crossing way given that it is only an informal crossing, so the separate sidewalk is not connected.

Do we expect the "generate corssings" step to add the missing link here?
I kind of did but I am not sure…

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Related https://github.com/a-b-street/speedwalk/issues/70