Add exe ghc-supported-languages

[philderbeast]

Fix for #11633.

Note

Should we move this module to cabal-dev-scripts?

Current output:

$ ghc --numeric-version
9.14.1

$ which ghc | xargs cabal run Cabal-tests:lang-tests
Warning: this is a debug build of cabal-install with assertions enabled.
The following extensions are known to GHC but are not in the 
extension registry in Language.Haskell.Extension.
  Haskell98
  Haskell2010
  GHC2021
  GHC2024
  ExplicitLevelImports
  NoExplicitLevelImports
  ImplicitStagePersistence
  NoImplicitStagePersistence
If these extensions are ready for public consumption then they 
should be registered. If they are still experimental and you 
think they are not ready to be registered then please add them 
to the exceptions list in this test program along with an 
explanation.

---

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)

[Bodigrim]

GHC has a specific notion of "experimental" for extensions whose semantics is not entirely set in stone. It is not an excuse for Cabal not to recognise them, so I think the wording here is confusing.

If an extension is available from a released GHC (or would be available soon as witnessed by an alpha release or RC), it must be recognised by Cabal, that's it. If one is testing an experimental build of GHC (such as GHC HEAD), the extension could remain unrecognised by Cabal for time being. Could you simplify the message to this effect?

[philderbeast]

Could you simplify the message to this effect?

Happy to.