Add GitHub actions

[alexsoft]

I guess actions themselves should be enabled in this repository, but I prepared two workflows:

1. run tests on different versions of php and with different flags (prefer-lowest and prefer-stable)
2. run tests on php 7.4 with coverage, as there is no need to execute coverage parts on all versions of php

[irazasyed]

Thanks for the PR. This was a much needed one. Appreciate it. 👍

Could you also please add PHP 8 support?

[alexsoft]

@irazasyed, sure.
Which version should be used for coverage workflow then? 7.4 or 8.0?

[irazasyed]

8.0 is good.

[alexsoft]

@irazasyed done. I also took the liberty to allow phpunit 9 :)

[irazasyed]

I noticed that. Good, appreciated :)

Thanks!

[alexsoft]

@irazasyed There is still something wrong with scrutinizer, but I am not absolutely sure how to fix it