Note that those are not documented, but based on trial-and-error.
Terrain and Watercolor return 404 at the next zoom level, indicating
that this is the max that they support.

Toner does serve some tiles at level 18, but a lot of them return a 503.

Since we aliased the field layer_type to layer, we would need to use
type=... to set it. However, it is a bit confusing if we access it as
TileLayerConfig.layer_type, but set it with type=..., therefore let's
just turn on allow_population_by_field_name. This option lets us use
layer_type=... to set the field when initializing the object.