Handle informal sidepath

This is just to document the case. I don't have an suggestion on how to handle this; nor do I think it's important to handle now.

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Sometimes there are informal sidewalks next to a road.
If those are mapped, marking them as `informal=yes` (DE: Trampelpfad) is a feasable tagging.

I am not clear on what the right `sidewalk`-value be in this case. It seems similar to the situation of https://trafficsigns.osm-verkehrswende.org/DE?signs=DE:239,1022-10 where bikes are allowed to use the (pedestrian) sidewalk (at walking speed) but this is not considered bike infra so it still gets a cycleway:SIDE=no on the centerline.

For the sake of routing, it would be better to recognize those `informal=yes` "`is_sidepath`" as some kind of infra and not generate a virtual sidewalk.

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Examples

* https://www.openstreetmap.org/way/1007134506/history
(Note I just changed the tagging to improve it a bit: better highway class `footway` but especially `is_sidepath`)

* There are more cases in this general area where the "sidewalk" is a dirt path
https://www.openstreetmap.org/way/4935223 In this case someone marked them as `sidewalk=both`

この issue では、OSM における非公式な側道のタグ付けの曖昧さについて説明しています。コードベースを理解するため、歩道生成のロジックと、`is_sidepath` および `informal=yes` タグが現在どのように扱われているかを調べてください。リポジトリのコードを確認し、歩道のルーティングとタグの解釈を調査してください。目的は解決策を提案することであり、実装することではありません。そのため、類似したタグの既存の扱いを調査し、仮想的な歩道の生成をどのように調整できるかを検討してください。

索引モデルが issue の本文から書いたものです。