Distinguish on_string_data and on_key_data

include/weaseljson.h

@@ -13,6 +13,11 @@ struct WeaselJsonCallbacks {
    * incomplete and there will be another call, potentially with more data
    */
   void (*on_string_data)(void *userdata, const char *buf, int len, int done);
+  /** The key data provided has already been unescaped, unless the
+   * WeaselJsonRaw flag is used. If `done` is false, this key may be
+   * incomplete and there will be another call, potentially with more data
+   */
+  void (*on_key_data)(void *userdata, const char *buf, int len, int done);
   void (*on_begin_array)(void *userdata);
   void (*on_end_array)(void *userdata);
   /*If `done` is false, this number may be incomplete and there will be another