Enable strict mode on assets

https://symfony.com/blog/new-in-symfony-5-4-misc-features-part-1#strict-mode-for-missing-assets

Author: sayjun0505

config/packages/assets.yaml

@@ -1,3 +1,4 @@
 framework:
     assets:
         json_manifest_path: '%kernel.project_dir%/public/build/manifest.json'
+        strict_mode: '%kernel.debug%'