Basemap refactor

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](https://leaflet-extras.github.io/leaflet-providers/preview/) (referenced from [here](https://python-visualization.github.io/folium/latest/getting_started.html#Choosing-a-tileset)). 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](https://xyzservices.readthedocs.io/en/stable/api.html#xyzservices.TileProvider).