This implements an initial GitHub Actions workflow for us - to be run on PRs and on `main` commits.
This just implements a really simple `style/fmt` check - running `prettier` on the `README.md`.
I assumed we'll stick with using a top-level `Makefile` for commands like in `m` and `link` - but open to alternatives, too!
Since I was adding a `package.json` and `node_modules` for this, I realized we were missing `.gitignore`s, so I added some a subset of the ignore files from `coder/m`