Fix potential exceptions in initialization script (#53)

Try to fix #50 

```release-note
修复搜索组件初始化脚本可能执行失败的问题。
```

Author: ruibaby

src/main/java/run/halo/search/widget/SearchWidgetHeadProcessor.java

@@ -35,7 +35,7 @@ private String searchWidgetScript() {
 
         return PROPERTY_PLACEHOLDER_HELPER.replacePlaceholders("""
             <!-- PluginSearchWidget start -->
-            <script src="/plugins/PluginSearchWidget/assets/static/search-widget.iife.js?version=${version}" async></script>
+            <script src="/plugins/PluginSearchWidget/assets/static/search-widget.iife.js?version=${version}" defer></script>
             <link rel="stylesheet" href="/plugins/PluginSearchWidget/assets/static/style.css?version=${version}" />
             <!-- PluginSearchWidget end -->
             """, properties);