Basic VSCode dev container setup for Haskell

See https://code.visualstudio.com/docs/devcontainers/containers for more information about dev containers.

This comes with:

• GHC (through GHCup)
• Cabal (through GHCup)
• hlint
• and stylish-haskell

It should be pretty easy to adapt to your needs but let me know if I can help.

N.B.

At the moment, there's no official Haskell dev container configuration but if this ever changes, please use the official setup.