Use Webpacks new thread-loader to improve build times

[swernerx]

Webpack has a new thread-loader which should be able to parallelize loader execution. This should in theory improve build times - especially in larger applications where a lot of code is local (loaders are mainly relevant for own code - not for 3rd party dependencies.)

See also: https://medium.com/webpack/typescript-webpack-super-pursuit-mode-83cc568dea79