[TASK] add extension key to composer.json

composer.json

@@ -22,6 +22,11 @@
     "replace": {
         "sgalinski/sg_mail": "self.version"
     },
+    "extra": {
+        "typo3/cms": {
+            "extension-key": "sg_mail"
+        }
+    },
     "autoload": {
         "psr-4": {
             "SGalinski\\SgMail\\": "Classes/"