validator warning when route=bicycle uses bicycle=use_sidepath ways

[Polyglot]

While working on some bicycle routes, it occurred to me that we have ​bicycle=use_sidepath now, which means there is a cycleway next to the road. It seems obvious that marked bicycle routes should prefer to use those cycleways.

This is a relatively simple thing to check for using MapCSS, but I'm not really sure where to add it.

[Klumbumbus]

In 12773/josm:

fix #15208 - warn when route=bicycle relation uses bicycle=use_sidepath ways