RFC: hiking_visibility key 🍿 paired with hiking_technique and hiking_exposure/fall_risk this rounds out the hiking path trinity

Some expanding on the comment above @ Should we just use `smoothness` and `surface` along with `sac_scale` on multi-use paths and trails instead of making new keys?

I do think this needs to be condensed a bit, the initial proposal was a very quick rough draft after I was self-isolating from extended COVID exposure while visiting elderly relatives.