Add point_count_abbreviated property to clustered features

[oreilles]

Add point_count_abbreviated property to clustered features to improve consistency with clusters generated by supercluster.
The formula used is taken from supercluster/index.js#L377-L379:

    const abbrev =
        count >= 10000 ? `${Math.round(count / 1000)  }k` :
        count >= 1000 ? `${Math.round(count / 100) / 10  }k` : count;

[e-n-f]

Could you please make this PR against https://github.com/felt/tippecanoe instead? The mapbox repo is essentially unmaintained now.

[oreilles]

Done 👍