Add a miles or kilometers config option for directions

[trosel]

Just as a guess, I think the option could be added in a similar spot to this userLang config option.

https://github.com/QwantResearch/erdapfel/blob/ffd91d287130c13620ca3565466e25bbd6d0acea/src/adapters/direction_api.js#L44

Something like const userUnits = masq.getPreferredUnits(); maybe?