Scripts: Clean up the build folder via clean-webpack-plugin

[ocean90]

Is your feature request related to a problem? Please describe.
Currently, when using a webpack config with multiple entry points and removing one, the old build files for the removed entry point remain in the build folder.

Another case is source maps which are created during the watch task but not used in the production builds.

Describe the solution you'd like
The webpack documentation recommends to use the popular clean-webpack-plugin plugin. We'd have to add it as a new dependency and adjust the plugins config to include new CleanWebpackPlugin().

Describe alternatives you've considered
Adding a prebuild npm script which cleans up the folder.

[ocean90]

Related to #23127 which renames style.css to style-index.css