Set up a CI pipeline with GitHub's actions

[reimic]

Automate the CI process with GitHub actions:

• step running unit tests for PHP == 7.4,
  (as an intermediate step Downgrade to PHP 7.4, move from Pest to PHPUnit, establish CI #25)
• step running unit tests for PHP == 7.0,
  (finally done with Run PHPUnit on PHP >= 7.0 #86)
• step with matrix unit tests for all versions > 7.0,
• step with matrix unit tests for all versions > 7.4,
  (as an intermediate step Downgrade to PHP 7.4, move from Pest to PHPUnit, establish CI #25)
• step for automatic linting, after Downgrade codebase to 7.0 (to mirror WordPress' support range) #26
• step for coding standards check, after Downgrade codebase to 7.0 (to mirror WordPress' support range) #26