Webpack 5 Support

[naoak]

We have two PRs (developit/workerize-loader#101, developit/workerize-loader#114) for webpack 5 support in workerize-loader, but we can't test it unless we make karmatic webpack@5 compatible first. Now karma-webpack@5 supports webpack 5, so I think karmatic also could do it.

[developit]

Fixed in 507d488.

FYI - karma-webpack@5 has some major issues, but there's no alternative.