phpstan: Use standard config path

This allows developer to create their own own config file, e.g. for setting `editorUrl`:
https://phpstan.org/user-guide/output-format#opening-file-in-an-editor

Author: jtojnar

.gitignore

@@ -14,6 +14,7 @@ user.js
 *.ini
 node_modules
 .env
+phpstan.neon
 vendor/
 .php_cs.cache
 __pycache__