Improve Line Drawings for Map

[nimdanitro]

We want to improve the line-drawings for e.g. blocked/closed streets where we have to have an starting and ending icon.

Due to mapbox/mapbox-gl-js#9937 we can currently only add either a start or an end icon.

One option would be that for each corresponding line object, we dynamically decorate a corresponding icon object (on a separate layer?)

[nimdanitro]

see https://github.com/alantgeo/dataset-to-tileset/blob/master/index.js#L114 as inspriation