Separate release and dev builds of the online editor

Let `npm run build` do a release build and `npm run start` be used for
development.

tools/online_editor/webpack.dev.js

@@ -0,0 +1,10 @@
+const { merge } = require('webpack-merge');
+const common = require('./webpack.common.js');
+
+module.exports = merge(common, {
+    mode: 'development',
+    devtool: 'inline-source-map',
+    devServer: {
+        contentBase: './dist',
+    },
+});