update psalm/plugin-phpunit requirement from ^0.16.1 to ^0.18.4 (via #46)

Author: dependabot[bot]

composer.json

@@ -36,7 +36,7 @@
   "require-dev": {
     "jetbrains/phpstorm-attributes": "^1",
     "phpunit/phpunit": "^9.5.10",
-    "psalm/plugin-phpunit": "^0.16.1",
+    "psalm/plugin-phpunit": "^0.18.4",
     "squizlabs/php_codesniffer": "^3.6.2",
     "vimeo/psalm": "^4.15"
   },