Add support for -XGHC2021

This adds support for -XGHC2021, including setting it by default, and extending the user’s guide.

The main work here was updating the test suite. The test suite is affected in these ways:

• Some tests rely on NondecreasingIndentation. Some I have changed to not need this, some I have added {-# LANGUAGE NondereasingIndentation #-}, some I have added {-# LANGUAGE Haskell2010 #-}
• Some tests fail with (I think) PolyKinds. I have added {-# LANGUAGE Haskell2010 #-}
• Some SafeHaskell related tests fail because GeneralizedNewtypeDeriving is on by default. I have added {-# LANGUAGE Haskell2010 #-}.