Add Thunderbird landscape as a source for topographical map tile styles

[rudokemper]

Feature Request

Per @luandro, it would be good to have a topographical data source. This source suggests that we could use the Thunderforest landscape style:

https://tile.thunderforest.com/landscape/{z}/{x}/{y}.png?apikey=YOUR_API_KEY

Thunderforest has a Map API with a monthly tile limit of 150,000 tiles: https://www.thunderforest.com/docs/map-tiles-api/

We can add thunderforest as a style option, and define the style parameters in downloadOnlineTiles().

[rudokemper]

Once this is complete, we can file an issue in https://github.com/ConservationMetrics/map-packer to add thunderforest as an option in the style dropdown.