Supported extensions app not in any package

[philderbeast]

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, #11627 (comment)

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?