Infer dedent from where token after do block

[carymrobbins]

The following is valid Haskell, so it'd be nice for it to be supported by Frege. For now (as a workaround), I'm simply dedenting the where token by one character.

foo :: Monad m => m ()
foo = do
  x <- y
  return x
  where
  y = return ()

[mabre]

duplicate of #272
The code can be compiled with version 3.24.100.