Move to GHC 9.2

[aspiwack]

The upcoming version 0.2 will presumably be the last version where we will support GHC 9.0.

Moving to GHC 9.2 will let us benefit from the improved multiplicity inference and Core generation that we shipped there.

This issue is blocked on haskell/haskell-language-server#2179 (partially so: I think that hls will be working well enough with GHC 9.2 before this issue is closed) This issue is blocked on Stackage publishing a snapshot for GHC 9.2

• [ ] Change the default compiler in shell.nix to GHC 9.2
• [ ] Change the stack snapshot to a GHC 9.2 one
• [ ] Make sure the CI runs on GHC 9.2
• [ ] Remove the scrutinee & \case trick, both from the documentation and from the code