Show slight arrows on freeway lanes

[1ec5]

Deceleration lanes at exits and forks on freeways are currently marked by ordinary 90° left or right turn arrows, which can look strange next to a much gentler maneuver arrow:

Normally, the convention in OpenStreetMap is to tag freeway exits and forks with slight_left or slight_right turn lanes, but some localities signpost 90° turn lane signs at exits, too. Besides, we don’t even show a slight right turn icon when there’s a slight_right turn lane at an ordinary intersection, because navigation-ui-resources has identical artwork for arrow-30 and arrow-45: mapbox/navigation-ui-resources#8. A slight turn arrow could be reused inside the exit tab for #1262.

/cc @bsudekum @JThramer @frederoni @danesfeder