feat: add cabal-gild

[amesgen]

A(nother) .cabal file formatter:

• https://github.com/tfausak/cabal-gild
• https://taylor.fausak.me/2024/02/17/gild/

The nixpkgs-stable bump isn't strictly necessary, but it seems cleaner compared to writing ad-hoc code to handle the absence of cabal-gild in 23.11.