Include current bearing at point returned from @turf/along

I find that often when I call turf.along I also need the angle of the given point along the line (for instance, if animating an icon along a line, you want it to rotate to face the direction of travel).

There's no convenient way to compute that angle: you can use turf.bearing, but you have to compute a point slightly forward and slightly behind the point (two more calls to turf.along), and handle the cases where you're very close to the end of the line.

Either of these would be great:

• turf.along could provide that bearing as a property on the feature (maybe an optional flag, so the computation doesn't happen if it's not needed)
• a convenient turf.bearingAt method, taking a point and a line and returning the angle
• or maybe adding an extra property to the return value of turf.nearestPointOnLine, to include the angle of the line at that nearest point.

Thoughts?

Start by reading the @turf/along and @turf/nearestPointOnLine entry points named in the issue, then inspect how their returned features and properties are tested. Done requires choosing and documenting one bearing API, implementing it consistently, and adding coverage for points along a line, including positions near its end.