Allow modification of webpack config for custom workers

[gabteles]

Custom workers are bundled with fixed webpack configuration. This PRs allows the configuration to be changed. A concrete use case for it is the one I outlined in the examples folder: it may be necessary to set up resolve aliases for the workers to share files with the rest of the application.

[kyh]

can we get this merged?

[ImBIOS]

@gabteles @kyh It seems this repo is dead and the maintainer cannot be contacted for so long now. I've made an immediate response by resurrecting this repo ImBIOS/next-pwa.

[ITJesse]

@ImBIOS As I said before, you are stealing others' works. https://github.com/ImBIOS/next-pwa/tree/master/examples/custom-worker-webpack You copied their code without giving any credit.