restore clean-webpack-plugin

This should be revisited with a review of the production workflow.

clean-webpack-plugin has old, outdated dependencies and does the simple
task of cleaning build/ and docs/.  Production workflows should start
with those dirs empty and this plugin should be removed.  Development
users can wipe rm -rf build/* as needed, without excess plugin
dependencies.

config/webpack.config.js

@@ -1,7 +1,7 @@
 const constants = require('../src/js/constants.js');
 const configs = require('../src/js/configs.js');
 
-const CleanWebpackPlugin = require('clean-webpack-plugin');
+const { CleanWebpackPlugin } = require('clean-webpack-plugin');
 const CopyWebpackPlugin = require('copy-webpack-plugin');
 const exec = require('child_process').spawnSync;
 const HtmlWebpackPlugin = require('html-webpack-plugin');
@@ -17,6 +17,7 @@ const revision = () => {
 
 // the many plugins used
 const plugins = [
+  new CleanWebpackPlugin(),
   new HtmlWebpackPlugin({
     constants,
     configs,