Replace Travis with Github Actions

[ShawnCZek]

As requested in https://github.com/wohali/oauth2-discord-new/pull/45#issuecomment-1404962593, I have replaced Travis with GitHub Actions. The actions are inspired by other official OAuth 2.0 Client providers.

To make the code coverage work, you have to set up Codecov integration for this repository. Once you do that, update the badge in README as well, please, as I cannot handle this. Otherwise, all other actions should work out of the box.

[ShawnCZek]

Also, two other things should be considered before merging this:

1. What is the .scrutinizer.yml file for? It looks like the integration is not even configured for this repository.
2. Should we not check the code style against PSR-12 instead of the outdated PSR-2?

Though, when I am thinking of it now, these should be resolved in a different PR.

[wohali]

1. What is the `.scrutinizer.yml` file for? It looks like the integration is not even configured for this repository.

I think I had this set up locally via a pre-commit hook. We could add those to the repo if you like.

2. Should we not check the code style against PSR-12 instead of the outdated PSR-2?

PSR-12 (July 2019) was not even created when this project was born (October 2017). I'm open to updating coding standards if appropriate.

Both changes belong in a new PR, but they could be in the same PR structured as 2 commits (all intermediate changes squashed, please).

I hope to look at this PR tomorrow.

[ShawnCZek]

@wohali I have no experience with Scrutinizer. I would say that is up to you to either set it up or remove it from the repository.

We can switch to PSR-12 in a different PR so this PR brings only one change.

[ShawnCZek]

@wohali, hey, have you had any luck reviewing this PR? Or are there any other changes I should make? 👀

[wohali]

I have not, but I promise I am still here. I will get to this eventually... sorry for the delay.