Add phpstan for static analysis

[ctrl-f5]

• add phpstan checks in the pipeline, level 5
• add phpunit to require so we can validate version (issue phpunit versions should be constrained through composer.json #193)
  • set minimum required phpunit version to 8.2.3 (ReflectionType::__toString() is deprecated in PHP 7.4 sebastianbergmann/phpunit#3728)
• fix static analysis issues
• extends and contains PR fix type hints #192

[ctrl-f5]

github actions seem very unstable, composer install randomly fails with "access denied" in the file system or "resource unavailable"

edit: seems to be a known issue on the windows machines: ramsey/composer-install#79

[cfmack]

Acknowledged that issue is randomly failing. Thanks for finding the link. We moved to Github Actions after Travis changed their terms of use.