migrate from webpack-contrib

[alexander-akait]

Recently, we are increasingly getting rid of non-official loaders and plugins to reduce the time for support and maintenance. Unfortunately this loader is very specific so we decide to take away mocha-loader from webpack-contrib. Feel free to feedback

/cc @spalger You may be interested in support. I still will help you and ask your questions.

Otherwise, it will be deprecated in the near future without official replacement, I am sorry