fix: First item in list must be a paragraph (#8632)

closes #8611

closes #8216

shared/editor/nodes/CheckboxItem.ts

@@ -22,7 +22,7 @@ export default class CheckboxItem extends Node {
           default: false,
         },
       },
-      content: "paragraph block*",
+      content: "block+",
       defining: true,
       draggable: true,
       parseDOM: [

shared/editor/nodes/ListItem.ts

@@ -26,7 +26,7 @@ export default class ListItem extends Node {
 
   get schema(): NodeSpec {
     return {
-      content: "paragraph block*",
+      content: "block+",
       defining: true,
       draggable: true,
       parseDOM: [{ tag: "li" }],