Create GitHub reporter

[borama]

Hello, we want to use Slim Lint in our GitHub Actions CI pipeline so I would like to add a GitHub reporter. The name of it is taken from the Rubocop GitHub Actions formatter which does the same thing for Rubocop.

I'm also adding a sample GH Actions workflow snippet that follows the new Rails GH CI convention to the README. This setup creates Pull Request annotations on Lint failures that look like this:

[sds]

Thanks!

[tagliala]

Hello, is it possible to sideport this to Haml-Lint?

edit: oh great, it seems to be already there!