CLI: Support specifying input folders in addition to just input files

[matobet]

It would be nice if fourmolu could take folders as well as files as an input and would process all haskell files in that folder (recursively) so instead of

fourmolu -i $(find src -name '*.hs')

I'd be able to do just

fourmolu -i src

This is consistent what formatters in other languages support and would be a nice quality of life improvement.

[matobet]

I dared to take a stab at this in https://github.com/tweag/ormolu/pull/911. Feedback welcome.

[amesgen]

Thanks, but this is a duplicate of #691/#697.

In general, I think it is more composable to outsource listing files to another tool (find, fd, git ls-files), rather than adding such functionality to Ormolu.

[matobet]

For more advanced usages and patterns I definitely agree. But supporting the basic case as the "all *.hs files in this directory" seems to be a better UX overall and something a lot of users (including me) would expect to work out of the box. Just my $0.02.

[mrkkrp]

Closing as a duplicate of #691.