Add Cabal External Commands

Add Cabal External Commands

• Replace the README with a user guide.
• Add cabal external commands, dpack, ypack and diy-pack.
• Require hpack ^>= 0.39.6.
• Test with GHC in {9.2.8, 9.4.8, 9.6.7, 9.8.4, 9.10.3, 9.12.4}.

Installation

$ cabal install hpack-dhall --overwrite-policy=always --with-compiler=ghc-9.12.4 --allow-newer=dhall-json:aeson