Create a GitHub Action to run fawltydeps

[GuillaumeDesforges]

Make a GitHub Action so adding FawltyDeps to a CI on GitHub Actions is a one-liner in the YAML file.

Incorporate getting this published in GitHub Marketplace as contemplated in #332

[jherland]

As @vreuter suggest in #332, the GitHub Action should of course be published in the GitHub Marketplace too.

[obscurerichard]

I'm happy to take this on.

[obscurerichard]

The first cut at this is in https://github.com/tweag/FawltyDeps-action - I made it a separate repo following GitHub's guidelines in choosing a location for your action.

[jherland]

The first cut at this is in https://github.com/tweag/FawltyDeps-action - I made it a separate repo following GitHub's guidelines in choosing a location for your action.

Looks really good! 🎉

I found a couple of minor things while reviewing it: tweag/FawltyDeps-action#3

[GuillaumeDesforges]

The GitHub action is there, I'll mark as done. Thanks a lot @obscurerichard !