feat(cli): support site.headHtml option (#10807)

* feat(cli): support site.headHtml option

* docs: update

* docs: update

packages/vant-cli/site/index.html

@@ -15,6 +15,9 @@
     <meta http-equiv="Cache-Control" content="no-cache" />
     <meta http-equiv="Pragma" content="no-cache" />
     <meta http-equiv="Expires" content="0" />
+    <% if (headHtml) { %>
+      <%- headHtml %>
+    <% } %>
     <% if (baiduAnalytics) { %>
     <script>
       var _hmt = _hmt || [];

packages/vant-cli/site/mobile.html

@@ -15,6 +15,9 @@
     <meta http-equiv="Cache-Control" content="no-cache" />
     <meta http-equiv="Pragma" content="no-cache" />
     <meta http-equiv="Expires" content="0" />
+    <% if (headHtml) { %>
+      <%- headHtml %>
+    <% } %>
     <% if (enableVConsole) { %>
     <script src="https://unpkg.com/vconsole/dist/vconsole.min.js"></script>
     <script>