Provide a GitHub Action

[grahamc]

It'd be really great if I could hook up a GitHub Action with nixfmt so it provided annotations to the code about reformatting that needs doing.

[Lucus16]

I think that would be cool too, but I don't really know anything about Github actions. There is already the -c flag to check if files are formatted, for use in CI. If you just want a list of files that need formatting, it should be pretty trivial and I'd appreciate a PR. If github supports annotations per line, I think that would be easiest to achieve with a separate diff tool since nixfmt does not remember source locations after parsing.