Style statistics based on source layer

[ZeLonewolf]

This is a follow-on from #747 to document additional work improve the stats generator.

We could group the layers by source layer to get pretty similar statistics, modulo some layer organization weirdness in OpenMapTiles.

Originally posted by @1ec5 in #747 (comment)

[1ec5]

To implement this enhancement, replace:

openstreetmap-americana/scripts/stats.js

Line 45 in 09cf915

let layerGroup = layer.id.split("_", 1)[0];

with:

let layerGroup = layer["source-layer"] || layer.source || layer.type;

and revert the style layer IDs to what they were before #747.

This will help us see not only where the style can be optimized but also where an Americana-specific tile schema could be more optimal than OpenMapTiles.

[ZeLonewolf]

With the suggested change, it sounds like the layer IDs are irrelevant as long as they're distinct.