Add option to embed OpenHistoricalMap

This extension’s support for Leaflet and openstreetmap-carto raster tiles is useful for some sites, but others like Housetrails, which focuses on historical geography, would benefit from maps that place the overlays in the correct time period instead of the present day. OpenHistoricalMap is a complement to OpenStreetMap that focuses on mapping history. It would be great if a site like Housetrails could embed an OHM map instead of an OSM map when a date is provided as context.

Unlike openstreetmap-carto, the main OHM map uses vector tiles that are rendered on the client side using MapLibre GL JS, but there’s a plugin for bridging the rendered style into Leaflet. openhistoricalmap.org itself uses Leaflet with this plugin.

Start by tracing the extension's existing Leaflet and OpenStreetMap raster-tile embedding options. Review the OpenHistoricalMap vector-tile and MapLibre GL JS approach, including its Leaflet bridging plugin; done means a site can select an OHM map when a date is supplied as context.