chore: add Config namespace to appstarter composer.json

To suppress warning like this:
Class Config\Email located in ./app/Config/Email.php does not comply with psr-4 autoloading standard (rule: App\ => ./app). Skipping.

Author: kenjis

admin/starter/composer.json

@@ -20,7 +20,8 @@
     },
     "autoload": {
         "psr-4": {
-            "App\\": "app/"
+            "App\\": "app/",
+            "Config\\": "app/Config"
         },
         "exclude-from-classmap": [
             "**/Database/Migrations/**"