ビルドの進捗を表示する
ProgressPlugin
参考：
ProgressPlugin | Webpack [Official]
Reporting Progress – Plugin API | Webpack [Official]
webpack/ProgressPlugin.js at v4.25.1 · webpack/webpack – GitHub
Webpack progress using node.js API – Stack Overflow
progress-bar-webpack-plugin
参考：
clessg/progress-bar-webpack-plugin: A progress bar plugin for Webpack – GitHub