Add aboriginal lands (#1489)

This PR adds support for `boundary=aboriginal_lands` by adding it to the `park` layer. While these lands are certainly not "parks", they have similar treatment from a technology perspective and can benefit from the existing processing chain established in that layer. I set all of these objects to `class=aboriginal_lands` in the tiles, including at the lowest zoom in order to separate it from the protected area merging implemented in #1160. In order to distinguish these from general parks, I expose the `class` attribute for these objects at z4 and also ensure that the z4 generalization is performed separately for protected areas versus aboriginal lands.

This unblocks #ZeLonewolf/openstreetmap-americana#105, which describes why having indigenous land boundaries is an important general feature on the map. This is also consistent with my suggested implementation in https://github.com/openmaptiles/openmaptiles/issues/1296#issuecomment-967749403.

layers/park/park.yaml

@@ -17,6 +17,7 @@ layer:
           `nature_reserve` is the class of `protection_title=Nature Reserve` and `leisure=nature_reserve`.
           The class for other [`protection_title`](http://wiki.openstreetmap.org/wiki/key:protection_title) 
           values is similarly assigned.
+          The class for `boundary=aboriginal_lands` is `aboriginal_lands`.
     name: The OSM [`name`](http://wiki.openstreetmap.org/wiki/Key:name) value of the park (point features only).
     name_en: English name `name:en` if available, otherwise `name` (point features only).
     name_de: German name `name:de` if available, otherwise `name` or `name:en` (point features only).