Update list of available vector basemap styles

[gowin20]

The VectorTileLayer class has a BasemapStyles type that's trying to enumerate all of the basemap styles available from the service. If this type is going to be maintained, it needs to be updated as all "OSM" styles have been deprecated. They have been replaced by a new family of basemaps called open, e.g. open/streets, open/hybrid, etc.

Full info here: https://developers.arcgis.com/documentation/mapping-and-location-services/mapping/basemaps/open-styles/

P.S. All styles currently offered by the service can be obtained by making a /self request.

[slutske22]

Do you think it would be more useful to type this as a simple string? Or is the enum useful

[gowin20]

The enum is useful but also requires some overhead to maintain, as the list of styles changes somewhat frequently. It's up to you, but we've opted not to include enums like this in base esri-leaflet-vector because of the added maintenance work.