fix(mp): v-model support dynamic add reactive properties

packages/vue-cli-plugin-uni/packages/mp-vue/dist/mp.runtime.esm.js

@@ -5893,14 +5893,16 @@ function internalMixin(Vue) {
     if (!target) {
       target = this;
     }
-    target[key] = value;
+    // 解决动态属性添加
+    Vue.set(target, key, value)
   };
 
   Vue.prototype.__set_sync = function(target, key, value) {
     if (!target) {
       target = this;
     }
-    target[key] = value;
+    // 解决动态属性添加
+    Vue.set(target, key, value)
   };
 
   Vue.prototype.__get_orig = function(item) {