fix(create-app): should set css preprocessor

packages/create-vant-cli-app/generators/templates/src/demo-button/index.less

@@ -1,3 +0,0 @@
-.test {
-  color: red;
-}

packages/create-vant-cli-app/generators/templates/src/demo-button/index.vue

@@ -18,7 +18,7 @@ export default {
 };
 </script>
 
-<style lang="less">
+<style lang="<%= cssLang %>">
 .demo-button {
   min-width: 120px;
   color: #fff;

packages/create-vant-cli-app/generators/templates/src/style/base.less

@@ -1,5 +0,0 @@
-@import "./var.less";
-
-body {
-  color: @text-color;
-}

packages/create-vant-cli-app/generators/templates/src/style/var.less

@@ -1,2 +0,0 @@
-@primary-color: #f44;
-@text-color: #4a4a4a;

packages/create-vant-cli-app/generators/templates/vant.config.js

@@ -1,6 +1,9 @@
 module.exports = {
   name: '<%= name %>',
   build: {
+    css: {
+      preprocessor: '<%= preprocessor %>',
+    },
     site: {
       publicPath: '/<%= name %>/'
     }