enumerate the layers when using kart import

[rhysallister]

kart import, with no layer specified, gives a list of layers to import with a prompt to type the layer name that you would like to import. It would be useful if the list were enumerated and then the number could be used to indicate which layer is to be imported. This would be easier than typing the layer name (which may be long or possibly contain characters such as '🐘' that are not trivial to enter with a keyboard).