minor #14 declare Symfony Flex as an allowed plugin (xabbuh)

This PR was submitted for the 5.4 branch but it was merged into the 4.4 branch instead.

Discussion
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declare Symfony Flex as an allowed plugin

Commits
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8f0f0ca declare Symfony Flex as an allowed plugin

Author: xabbuh

composer.json

@@ -50,6 +50,9 @@
         "symfony/web-profiler-bundle": "^4.4"
     },
     "config": {
+        "allow-plugins": {
+            "symfony/flex": true
+        },
         "preferred-install": {
             "*": "dist"
         },