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.
