Add GitHub actions for CI

[duboism]

#2 ported the test suite to to PHPUniPolyFills. This PR adds CI in form of GitHub actions. This is another step on the road to port the code to PHP 8.

Technical details

• The test suite itself is not changed.
• We use Setup PHP Action to set up PHP.
• We test up to PHP 7.4 for now since the code doesn't run on PHP 8.
• We test under Ubuntu and macOS.
• The action is configured to run on pull requests and when pushing on certain branches.

[duboism]

Thanks for merging this.