Add a heatmap option

[alulsh]

At high zoom levels (z0-z4ish), a lot of markers in one area is cluttered:

We can solve this by adding the option to display as a heatmap (https://docs.mapbox.com/mapbox-gl-js/example/heatmap-layer/). Just like in the example, we can display a circle layer at higher zoom levels so that you can still see specific birth locations.

I prefer heatmaps over clustering for this use case. With clusters you have to zoom in a lot to see the birth locations, which can be annoying and frustrating.