chore(sourcemap): add npm run command for a build with sourcemap file

package.json

@@ -16,6 +16,7 @@
     "build:doc": "cross-env STATIC_FILES_SUBFOLDER=sheets npm run build",
     "build:webpack": "webpack --progress --colors --config webpack.prod.js",
     "build:webpack-dev": "webpack --progress --colors --config webpack.dev.js",
+    "build:webpack-sourcemap": "webpack --progress --colors --config webpack.sourcemap.js",
     "start": "webpack-dev-server --progress --colors --config webpack.dev.js",
     "fix-memory-limit": "increase-memory-limit"
   },

webpack.sourcemap.js

@@ -0,0 +1,8 @@
+var merge = require('webpack-merge')
+var common = require('./webpack.common.js')
+
+// will create a build plus separate .min.js.map source map for debugging
+module.exports = merge(common, {
+    devtool: 'source-map',
+    mode: 'development'
+})