Fix export strategy of turf library

[lukas-h]

I moved this to a separate, new issue:

     Exactly, so far none of the boolean functions have been exported at all. I have exported all boolean functions as part of booleanWithin #167 in `package:turf/boolean.dart`. But we need to take a closer look:

• package:turf/turf.dart should export all functionality, but a lot is missing (boolean functions among others)
• package:turf/intersection.dart is missing
• package:turf/invariant.dart is missing
• package:turf/line_intersect.dart is missing
• package:turf/line_slice.dart is defined as library turf_along. Seems to be an copy paste error.
• package:turf/extensions.dart and package:turf/cluster.dart are the only meta functionality directly exported.
• package:turf/meta.dart has all meta files except extensions.dart. Interesting, that meta also has export 'src/line_segment.dart' hide lineSegment;

Seems to have grown historically. Maybe we should take another look at it together? @lukas-h

Originally posted by @jsiedentop in #151 (comment)