- allow HLS to use Cabal instead of Stack
- Cabal works fine, and I do not see any advantage to using Stack
- someone using Stack should test this does not switch him to Cabal ⚠️
- ignore `cabal.project.local` so I can put there `ghc-options: -haddock`
- this flag could lead to errors before GHC 9, but it gives me [docs on hover](https://haskell-language-server.readthedocs.io/en/latest/features.html)
