Basemap refactor

[kylebarron]

The dict should be a valid mapbox style spec object.

We might also want to have helpers for a single-layer raster-tile basemap. E.g. lonboard.basemap.RasterBasemap which takes in an xyz url. We should have an example with standard OSM tiles.

There are a bunch of default public raster tile basemaps here (referenced from here). Also see https://github.com/geopandas/xyzservices, which may be a better integration point. We should be able to integrate directly with the TileProvider class.

[kylebarron]

It may also be useful to refer to these basemaps as MaplibreBasemap objects, where we could also have DeckglBasemap objects in the future, for applications that don't want to render Maplibre beneath deck.gl