Plugin in composer.json (#2502)

Per https://getcomposer.org/doc/06-config.md#allow-plugins, Composer has added an extra layer of security in V2.2, and will start enforcing it in July of this year. Phpcodesniffer uses a plugin and will be affected if composer.json isn't updated before that date.

Co-authored-by: Mark Baker <mark@lange.demon.co.uk>

composer.json

@@ -12,7 +12,10 @@
         "spreadsheet"
     ],
     "config": {
-        "sort-packages": true
+        "sort-packages": true,
+        "allow-plugins": {
+            "dealerdirect/phpcodesniffer-composer-installer": true
+        }
     },
     "homepage": "https://github.com/PHPOffice/PhpSpreadsheet",
     "type": "library",