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

该 issue 涉及在存在非正式过街点时，将一条 footway 连接到生成的 sidewalk。查看代码库中的 'generate crossings' 步骤，该步骤可能位于网络生成模块中。检查当前如何连接过街点，以及在哪里处理 footway 和 sidewalk 的几何图形。查看提供的 Mapillary 和 speedwalk 链接，以了解真实世界中的场景。确定算法是否应自动为非正式过街点创建连接，还是需要手动干预。

由索引模型根据 Issue 内容生成。