Feature Request: Cluster map markers

### Impacted plugin

Jetpack

### What

Nearby markers should be clustered for UX

### How

Currently nearby markers overlap each other, and are indistinguishable until you zoom in:

[MapBox supports clustering](https://docs.mapbox.com/mapbox-gl-js/example/cluster/), and IMO it'd be a good feature to add.

Start with Jetpack's map-marker implementation and compare its current behavior with the linked Mapbox clustering example. Confirm how nearby markers are represented and rendered, then verify that overlapping markers are grouped and become individually distinguishable as the map is zoomed; no source file or test is named in the issue.

Written by the indexing model from the issue text.