Haskell support

The Haskell ecosystem often suffers from outdated package bounds. Various attempts have been made to improve the situation, but ultimately developers need a convenient method to update their bounds.

Haskell uses [PVP](https://pvp.haskell.org/) for versioning (which is somewhat similar to semver). It then uses [Cabal](https://github.com/haskell/cabal/) for package and version management (similar to rusts cargo). Packages are shared and uploaded on [hackage](http://hackage.haskell.org/).

I think the main hurdle of adding support here is that the cabal file format is custom. Obviously there exists a haskell parser in the [Cabal library](http://hackage.haskell.org/package/Cabal). There also seems to be *some* [documentation on the file format](https://cabal.readthedocs.io/en/latest/buildinfo-fields-reference.html). I'm not sure if that's enough to re-implement a parser or if a regexp solution would be possible.

The issue names Cabal files, the Cabal library, and Cabal build-info documentation; start by reviewing those resources and existing Dependabot ecosystem implementations. Determine whether the requested Haskell package-bound updates can be specified and validated; the issue does not name a repository file or test, so the completion criteria need clarification.

Written by the indexing model from the issue text.