add getCharacterCount method to the editor

packages/core/src/Editor.ts

@@ -395,6 +395,13 @@ export class Editor extends EventEmitter {
     return JSON.stringify(defaultContent) === JSON.stringify(content)
   }
 
+  /**
+   * Get the number of characters for the current document.
+   */
+  public getCharacterCount() {
+    return this.state.doc.content.size - 2
+  }
+
   /**
    * Destroy the editor.
    */