fix: do not add link tag to demo page when css extract is false (#3351)

Author: houfeng0923

packages/@vue/cli-service/lib/commands/build/demo-lib-js.html

@@ -1,7 +1,9 @@
 <meta charset="utf-8">
 <title><%- htmlWebpackPlugin.options.libName %> demo</title>
 <script src="./<%- htmlWebpackPlugin.options.libName %>.umd.js"></script>
+<% if (htmlWebpackPlugin.options.cssExtract) { %>
 <link rel="stylesheet" href="./<%- htmlWebpackPlugin.options.libName %>.css">
+<% } %>
 
 <script>
 console.log(<%- htmlWebpackPlugin.options.libName %>)

packages/@vue/cli-service/lib/commands/build/demo-lib.html

@@ -2,7 +2,9 @@
 <title><%- htmlWebpackPlugin.options.libName %> demo</title>
 <script src="https://unpkg.com/vue"></script>
 <script src="./<%- htmlWebpackPlugin.options.libName %>.umd.js"></script>
+<% if (htmlWebpackPlugin.options.cssExtract) { %>
 <link rel="stylesheet" href="./<%- htmlWebpackPlugin.options.libName %>.css">
+<% } %>
 
 <div id="app">
   <demo></demo>

packages/@vue/cli-service/lib/commands/build/resolveLibConfig.js

@@ -63,7 +63,8 @@ module.exports = (api, { entry, name, formats }, options) => {
             template: path.resolve(__dirname, template),
             inject: false,
             filename: 'demo.html',
-            libName
+            libName,
+            cssExtract: config.plugins.has('extract-css')
           }])
     }