chore: configure CI in Github actions

[czosel]

• runs tests using GitHub actions
• re-enables eslint
• use jest coverage instead of coveralls

[czosel]

@ichiriac I usually prefer if all CI steps can be easily executed locally, because it allows quicker turnaround times and makes contributing easier. That's why I replaced coveralls with jest's coverage feature and I'd also like to propose removing codeclimate in favor of a stricter eslint config (if needed at all). What do you think?

[ichiriac]

👍 we can merge it

[czosel]

I'll go ahead and merge this so that we have a working CI again 🙂