Supported extensions app not in any package

While checking out a review comment, I noticed that this module, `Cabal-tests/tests/misc/ghc-supported-languages.hs`, is not included in any package, https://github.com/haskell/cabal/pull/11627#discussion_r2940845184

I added it as an executable to the Cabal-tests package and saw that it doesn't compile as-is.

Should we move it to the cabal-dev-scripts package?