Make it possible to restrict the number of explicit imports

[crtschin]

There isn't much support for styles where one imports unqualified. It'd be nice to have a hlint rule that tries to restrict the number of explicit imports so the user either imports qualified or unqualified, but not to have import lists 20-long, which is quite easy to get to in this HLS-age.

-- Given a setting "explicitImportLengthLimit: 3"
--
-- Would reject
import Control.Monad.State (get, put, state, modify, gets) 

-- Would accept
import Control.Monad.State (get, put, state) 
import Control.Monad.State qualified as State
import Control.Monad.State