Remove the unplugin dependency and implement native bundler plugins instead. Unplugin pins us to specific Node versions; removing it also allows us to use newer bundler features without dropping Webpack 4 support. **Rollup PoC:** [https://github.com/getsentry/sentry-javascript-bundler-plugins/pull/858](<https://github.com/getsentry/sentry-javascript-bundler-plugins/pull/858>)
