fix(build): add charset to demo pages, fix #1765 (#1793)

Author: haoqunjiang

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

@@ -1,3 +1,4 @@
+<meta charset="utf-8">
 <title><%- htmlWebpackPlugin.options.libName %> demo</title>
 <script src="./<%- htmlWebpackPlugin.options.libName %>.umd.js"></script>
 <link rel="stylesheet" href="./<%- htmlWebpackPlugin.options.libName %>.css">

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

@@ -1,3 +1,4 @@
+<meta charset="utf-8">
 <title><%- htmlWebpackPlugin.options.libName %> demo</title>
 <script src="https://unpkg.com/vue"></script>
 <script src="./<%- htmlWebpackPlugin.options.libName %>.umd.js"></script>

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

@@ -1,3 +1,4 @@
+<meta charset="utf-8">
 <title><%- htmlWebpackPlugin.options.libName %> demo</title>
 <script src="https://unpkg.com/vue"></script>
 <script src="./<%- htmlWebpackPlugin.options.libName %>.js"></script>