[TIMOB-27478] Webpack: Use thread-loader in production builds

To speed up production builds we should include [thread-loader](https://github.com/webpack-contrib/thread-loader) before other expensive loaders, like babel-loader. The thread-loader is currently having some issues with platform specific files which needs to be investigated. We override Webpack's default compiler input filesystem with [PlatformAwareFilesystem](https://github.com/appcelerator/titanium-webpack-devkit/blob/a6af9bea700d2aeaa776defcbea96fe573047060/packages/webpack-target-titanium/src/plugins/PlatformAwareFileSystemPlugin.js#L12) to easily switch out files based on the current platform. For example, a require to @/components/Platform.vue uses @/components/Platform.ios.vue if available. This works fine in other loaders because they use the internal compiler filesystem. However, therad-loader has no access to that in workers, so it resorts to Node's [fs](https://github.com/webpack-contrib/thread-loader/blob/488300f5ad86c2eb794848de4cbc71c0cc5637da/src/worker.js#L107) module.