Add the option to display a map scale in the lower-left corner of Leaflet maps

[TroublesomeCorvid]

According to the official documentation, the functionality to display a map scale already exists in Leaflet: https://leafletjs.com/reference.html#control-scale

Please add an option to display a map scale to Leaflet maps displayed by the Maps extension.

If this option already exists, please update the documentation to describe how to activate it.