[TASK] Add extra.typo3/cms.extension-key to composer.json

In the next release of typo3/cms-composer-installers the
configuration of the extension key is enforced:

See: TYPO3/CmsComposerInstallers@3c60d05

composer.json

@@ -47,6 +47,7 @@
             "dev-master": "1.0.x-dev"
         },
         "typo3/cms": {
+            "extension-key": "extpagetitle",
             "app-dir": ".Build",
             "web-dir": ".Build/web"
         }