Allow loading dark tiles

[hlfan]

Implementing dark mode tiles in openstreetmap/openstreetmap-website#5328 requires expanding the layer definitions in leaflet-osm.js.
A possible solution is to add suffixes in the URLs based on a flag in the options.
For the current two layers that have dark mode variants, this could be achieved by adapting a few keys in the options objects.
Updating the tests would also be required.

[AntonKhorev]

Why can't their dark variants have another url like https://github.com/openstreetmap/openstreetmap-website/pull/4777/files#diff-4fe14e869cf17b068e1bea009a55b094fed13f0cad85337cf4769f32b40a01b5R262 ?

[tomhughes]

That does seem like a better solution to me.

[hlfan]

Yes! This is much better!

[hlfan]

Leaving this open as the code from the website repo may be propagated here too when an implementation gets merged.

[hlfan]

Closing in favor of a more modular solution as in ebfaf22.