This PR adds indigenous lands to the `boundary` layer by adding new area features for these objects, along with generalization up to z4. It borrows heavily from the technical processing chain of the `park` layer.
I added new `class`, `name`, and `name:xx` attributes to the boundary layer, in order to support the new area features. Should we (or someone extending this schema) come up with new objects to place in the boundary layer in the future, `class` would support additional categories.
This PR adds state name to national borders (`admin_level=2`)
From zoom level 3 there is an added `adm0_l` and `adm0_r` attribute for national borders.
There is 3 main steps:
1) union border lines from OSM and keep separate LineString between intersects
2) create polygons from step 1), create a point in each state polygon, add to this point a state abbreviation from NaturalEarth Data
3) create a short parallel line on the left and right of a small part of the border in 70m distance, and add information about which state overlap this short line.
