Allow to build with cabal

The process to create these files is roughly:

1. run `stack2cabal`
2. work around cabal bugs (see haskell/cabal#5444)
3. disable tests to avoid build failures with --enable-tests from
   unrelated packages (cabal considers all packages defined via
   source-repository-package as *local* and will try to build its tests)
4. fix package constraints in cabal.project.freeze, where stackage
   constraings were overwritten in stack.yaml files (usually raising
   a version number)

Rationale:

1. good to support all available tools as options for interested users
2. cabal allows easier managing of package bounds (and testing against
   latest package versions)
3. #1303 is an excellent workaround for nix users, but requires nix.
   This should be the go-to fallback in case cabal project files
   get out of sync for too long.