chore: set up phpstan

GitOrigin-RevId: 2682b4b92d71e4a1f05978fda914bbe240269caa
diff --git a/.gitignore b/.gitignore
index ece1035..87506c0 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,3 +3,4 @@
 favicon.ico
 custom
 files
+vendor