Proposal: export liftA2 from Prelude

[Andreas Abel]

Proposal: export liftA2 from Prelude.
More information and discussion here:

   https://gitlab.haskell.org/ghc/ghc/-/issues/20945

Discussion period 2 weeks

TL;DR

- Fixes weird qualification in warning

     • No explicit implementation for
         either ‘<*>’ or ‘GHC.Base.liftA2’
     • In the instance declaration for ‘Applicative <Foo>’

- Was considered for GHC 8.2 but never followed up