chore: set up phpstan

GitOrigin-RevId: 2682b4b92d71e4a1f05978fda914bbe240269caa
diff --git a/composer.json b/composer.json
new file mode 100644
index 0000000..2216211
--- /dev/null
+++ b/composer.json
@@ -0,0 +1,5 @@
+{
+    "require-dev": {
+        "phpstan/phpstan": "^1.10"
+    }
+}