'OSM Mapnik' bundled/default base map into 'openstreetmap' ('label' => 'OpenStreetMap')

Italo Mairorequested to merge
issue/leaflet-3586903:3586903-basemap_from_OSM_Mapnik_into_OpenStreetMap into 10.4.x

Leaflet module refactoring to change 'OSM Mapnik' bundled/default base map into 'openstreetmap' ('label' => 'OpenStreetMap')

Closes #3586903

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