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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
I like webpack just fine. I think roll up can get complicated with all the plugins. If it helps check out my typescript starter template on GitHub: https://github.com/bozdoz/template-webpack-typescript
Give a chance to jsenv. I have built this tool exactly to author project in es6 targeting browsers and/or Node.js. You will get the dev server + test runner + build script (minification and so on) compatible with es6: https://github.com/jsenv/jsenv-core.