Update to work with leaflet 10.2.24 and leaflet_countries

Problem/Motivation

I'm trying to get https://www.drupal.org/project/leaflet_countries → working with the latest leaflet module, currently 10.2.24

I think the leaflet module has evolved a bit, so we need some tweaks to leaflet_topojson to be able to support leaflet_countries with the latest leaflet module.

Starting this issue for further discussion and I'll create an issue fork to experiment with code.