Add support for specifying a working directory

[jkeuhlen]

fourmolu seems to inherit extension information from the directory where it is launched. For us, this means that running this action in a shared mono-repo "works", but it's missing things like a default TemplateHaskell extension setting and so it misapplies a large portion of formatting.

e.g. our project layout is something like
repo
| - .github
| - workflows
| - haskell-backend
| - typescript-frontend

If you run the exact same commands from the haskell-backend directory, everything works great.

GHA does support a working-directory flag for all run: blocks: docs. That has to be implemented at the library level though to combine it with anything that "uses" a library action. Could something similar be added to this action that essentially does a cd into the directory where it should run from first?

[cdepillabout]

That sounds reasonable. I'd be willing to merge in a PR implementing this.

[jkeuhlen]

Great. I ended up realizing that I didn't actually need this for my project (we just were failing to check in our .cabal file 🤦 ), but I had the workflow already set up and working so I figured I'd add it anyways in case it's useful in the future: #29