Fixing bug with clean-webpack-plugin deleting files in watch mode

Hi friends!

This release disables the cleanStaleWebpackAssets in clean-webpack-plugin by default to fix a bug where - in watch mode - some images or fonts would be incorrectly deleted. See #772 - thanks to @weaverryan.

To upgrade:

1. Update your package.json file: set the @symfony/webpack-encore version to ^0.30.2.

2. Run yarn upgrade!

v0.30.1...v0.30.2

Happy packing!