basic Y.Text toJSON returns unformatted string

This avoids text nodes in nested structures returning undefined when toJSON is called by a parent.

src/types/YText.js

@@ -668,6 +668,17 @@ export class YText extends AbstractType {
     return str
   }
 
+  /**
+   * Returns the unformatted string representation of this YText type.
+   *
+   * Avoids undefined being returned when called as a nested type.
+   *
+   * @public
+   */
+  toJSON () {
+    return this.toString()
+  }
+
   /**
    * Apply a {@link Delta} on this shared YText type.
    *