fix(webpack): use production mode on production

Author: hiroppy

packages/fusuma/src/webpack/webpack.config.js

@@ -59,6 +59,7 @@ module.exports = (
       ],
     },
     resolve: {
+      mainFields: ['module', 'main'],
       alias: {
         // https://github.com/facebook/react/issues/13991
         // for dev

packages/fusuma/src/webpack/webpack.prod.config.js

@@ -7,8 +7,7 @@ const css = require('./css');
 
 function prod() {
   return {
-    // mode: 'production',
-    mode: 'none', // if specify production, webpack won't generate dist directory(and mini-css-extract has errors)
+    mode: 'production',
     output: {
       filename: '[name].[chunkhash].bundle.js',
       chunkFilename: '[name].[id].[chunkhash].bundle.js',