Add a contextual layer with crossings

Over in https://a-b-street.github.io/severance_snape/ I've been playing with displaying crossings over main roads and measuring the distance between crossings. There are some longer-term ideas for incorporating crossings and "area porosity" into the LTN tool more substantially, but... a simple and immediately useful thing would just be a new layer, showing crossings scraped from the OSM data. It could either show all crossings (with styling/popups for details about the crossing) or only show over main roads. Maybe since the user can reclassify main roads here, showing all would be simplest.

It then helps someone to understand the value of reducing shortcuts on a particular street, if there's a crossing over the main road into an adjacent LTN.