Distinguish on_string_data and on_key_data

src/json_value.h

@@ -79,6 +79,18 @@ inline WeaselJsonCallbacks readValueCallbacks() {
       state->on_end_value();
     }
   };
+  result.on_key_data = +[](void *p, const char *buf, int len, int done) {
+    auto *state = (ReadValueState *)p;
+    if (!state->startedData) {
+      state->startedData = true;
+      state->valueStack.emplace_back(std::string());
+    }
+    std::get<std::string>(state->valueStack.back()).append(buf, len);
+    if (done) {
+      state->startedData = false;
+      state->on_end_value();
+    }
+  };
   result.on_begin_array = +[](void *p) {
     auto *state = (ReadValueState *)p;
     state->valueStack.emplace_back(std::make_unique<JsonArray>());