core needs preact

packages/core/package.json

@@ -6,6 +6,7 @@
   "description": "Provides core functionality, including the Calendar class",
   "docs": "https://fullcalendar.io/docs/initialize-es6",
   "dependencies": {
+    "preact": "^10.0.5",
     "tslib": "^1.9.3"
   }
 }