Basemap follow ups

[kylebarron]

• Validation for beforeId. If beforeId is passed to a layer, try to resolve the basemap style URL to a dict, and then validate that the string value of beforeId exists in that layer stack. Keep a cache of fetched styles so that you're not fetching them repeatedly.
• Warn if beforeId is set when the basemap style is not "interleaved"?
• (next PR): integration with https://github.com/geopandas/xyzservices (see Basemap refactor #494)
• (possibly future PR): examples of rendering different basemap modes

from #935