Use `cabal-gild`'s formatting for output

[neilmayhew]

cabal-gild does a great job of formatting .cabal files, and many projects run it automatically using GitHub Actions or local git hooks. It's a good representation of what most people would like their cabal files to look like.

The formatting function is defined here and is exposed here as part of the stable API.

[dan-t]

If the format is better, then it should be added to the cabal library, where it‘s maintained.

I‘m not eager to add a dependency that might not be maintained in the future, because the cabal library will for sure change.