move dropcursor and gapcursor to its own package

packages/starter-kit/index.ts

@@ -1,3 +1,5 @@
+import Dropcursor from '@tiptap/extension-dropcursor'
+import Gapcursor from '@tiptap/extension-gapcursor'
 import Document from '@tiptap/extension-document'
 import Paragraph from '@tiptap/extension-paragraph'
 import Text from '@tiptap/extension-text'
@@ -17,6 +19,8 @@ import ListItem from '@tiptap/extension-list-item'
 
 export default function defaultExtensions() {
   return [
+    Dropcursor(),
+    Gapcursor(),
     Document(),
     History(),
     Paragraph(),