add relevant type information

src/structs/ContentType.js

@@ -44,6 +44,9 @@ export class ContentType {
    * @param {AbstractType<YEvent>} type
    */
   constructor (type) {
+    /**
+     * @type {AbstractType<any>}
+     */
     this.type = type
   }
   /**

src/structs/Item.js

@@ -138,6 +138,9 @@ export class Item extends AbstractStruct {
      * @type {Item | null}
      */
     this.redone = null
+    /**
+     * @type {AbstractContent}
+     */
     this.content = content
     this.length = content.getLength()
     this.countable = content.isCountable()