[FEATURE] Add a PHPUnit-specific extension to PHPStan (MyIntervals#444)

This enables PHPStan to understand certain PHPUnit-specific constructs.

Author: oliverklee

composer.json

@@ -28,7 +28,9 @@
     },
     "require-dev": {
         "codacy/coverage": "^1.4.3",
+        "phpstan/extension-installer": "^1.3.1",
         "phpstan/phpstan": "^1.10.57",
+        "phpstan/phpstan-phpunit": "^1.3.15",
         "phpunit/phpunit": "^8.5.36"
     },
     "suggest": {