add capability to inspect a cluster on click to cluster example

[andrewharvey]

Launch Checklist

• briefly describe the changes in this PR

Methods for inspecting GeoJSON clusters were introduced in #6829, and now in 0.47.0. This PR adds to the cluster example the getClusterExpansionZoom method so when you click on a cluster the map zooms in to expand it.

• n/a write tests for all new functionality
• n/a document any changes to public APIs
• n/a post benchmark scores
• manually test the debug page
• n/a tagged @mapbox/studio and/or @mapbox/maps-design if this PR includes style spec changes

I wasn't able to get the shorthand click features to work, not sure why. 🤔

 map.on('click', 'clusters', function (e) {
    console.log(e.features)
}