Store a leaflet map instance on a DOM element so other plugins can write on the same map

[etjones]

On map creation, store the leaflet map instance on the DOM element .leaflet-container as datasetteClusterMap. Another plugin can then use that instance to write to the same map, getting the benefits of datasette-cluster-map's logic without.

Another plugin can then do

const mapDiv = document.querySelector(".leaflet-container");
if (mapDiv){
    const map =  mapDiv.datasetteClusterMap;`
}

I guess that this allows plugins to the datasette-cluster-map plugin. Yo dawg...

Includes a sanity-check test to verify that the handle is available