Add support for output.filename function

[jantimon]

It is possible to specify a function for webpack output filename.

https://webpack.js.org/configuration/output/#output-filename

Unfortunately this does not work with the ScssConfigWebpackPlugin because it tries to replace the name directly.

https://github.com/namics/webpack-config-plugins/blob/9cba7d7b66bcd6c58e2391ee9a772cdd0efab181/packages/scss-config-webpack-plugin/src/ScssConfigWebpackPlugin.js#L34-L38