build(deps-dev): update terser-webpack-plugin requirement (#211)

Updates the requirements on [terser-webpack-plugin](https://github.com/webpack-contrib/terser-webpack-plugin) to permit the latest version.
- [Release notes](https://github.com/webpack-contrib/terser-webpack-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/terser-webpack-plugin/blob/master/CHANGELOG.md)
- [Commits](webpack/terser-webpack-plugin@v1.4.1...v2.1.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

packages/superset-ui-plugins-demo/package.json

@@ -49,7 +49,7 @@
     "@babel/plugin-syntax-dynamic-import": "^7.2.0",
     "babel-loader": "^8.0.5",
     "gh-pages": "^2.0.1",
-    "terser-webpack-plugin": "1.4.1"
+    "terser-webpack-plugin": "2.1.0"
   },
   "peerDependencies": {
     "@superset-ui/chart": "^0.12.0",