conditionally add MiniCssExtractPlugin, supress missing TS export warnings

[robin-drexler]

• Removes warnings for missing TS exports
• adds MiniCssExtractPlugin if it doesn't exist to allow consumers to create a production bundle without having to do that themselves. Otherwise the build fails with an error. Caveat: the check will fail when the consumer uses a different version of MiniCssExtractPlugin, but I guess in that case we'll have other issues anyways.