Upgrade PHP and dev dependencies

/!\ Removed PHP 7.2 support /!\

- PHPUnit 8 => 9 (+ fix the tests)
- PHP-CS-Fixer 2 => 3 (+ fix the code)
- Introduced PHP stan

.gitignore

@@ -5,5 +5,5 @@
 /tests/coverage
 /vendor
 /composer.lock
-/.php_cs.cache
+/.php-cs-fixer.cache
 /.phpunit.result.cache