"Mine exclusively" toggle on excavators

A toggle near the "Mining priority" section on excavator UIs that, when enabled, will change its behaviour from "mine this material first then anything else later" as it is now to "mine only this material, ignore any designations that do not contain it". This will allow you to, for example, set a much more aggressive designation around a patch of coal and your excavators will warn you when the coal's dry rather than continuing to clear the surrounding rock and dirt, or to easily dig out a section of exposed rock and replace it with dirt without also stripping the dirt itself.