cluster feature add a extent's field

Hi, I use tippecanoe to get vector tile. Clustering point is very cool. I think maybe: extent of points property is need in clustering point properties. If that i can fitbounds to the extent when i click clustering point.

No source file or test is named. Start by locating the clustering-point property generation in tippecanoe and determine how an extent should be represented in clustered point properties; done means a cluster exposes the extent needed to fit bounds, with tests covering the resulting properties.

Written by the indexing model from the issue text.