Registry of special attributes

[e-n-f]

We should track these and make sure they are handled uniformly and perhaps give them a more native representation.

• clip_start, clip_end, lineMetrics in Calculate distance metrics for line features geojson-vt#93
• cluster, point_count, cluster_id in https://github.com/mapbox/supercluster
• clustered, point_count, sqrt_point_count in Add an option for clustering features tippecanoe#511
• tippecanoe_feature_density in Add an option to calculate feature density as a feature attribute tippecanoe#252

Others?

cc @mourner

[mourner]

Supercluster also has point_count_abbreviated. lineMetrics isn't an attribute — it's an option to the tiling library.

[e-n-f]

These could be represented as private-use messages on the feature in the style of #107