fix: missing css source map at dev environment (#460)

Author: sakokazuki

lib/webpack/createBaseConfig.js

@@ -197,7 +197,8 @@ module.exports = function createBaseConfig ({
         .options({
           modules,
           localIdentName: `[local]_[hash:base64:8]`,
-          importLoaders: 1
+          importLoaders: 1,
+          sourceMap: !isProd
         })
 
       rule.use('postcss-loader').loader('postcss-loader').options(Object.assign({