Support for webpack multiple-configurations

[Gozala]

We would like to produce separate dist of JS-IPFS that would allow sharing node across multiple browsing contexts through SharedWorker API (Contex: ipfs/js-ipfs#3022).

As far as I understand we need webpack with multiple-configurations, new one that targets webworker.

However it appears that using webpack config with multiple configurations is rejected by aegir

aegir build
Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
 - configuration[0] has an unknown property '0'. These properties are valid:
   object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtool?, entry?, externals?, infrastructureLogging?, loader?, mode?, module?, name?, node?, optimization?, output?, parallelism?, performance?, plugins?, profile?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolveLoader?, serve?, stats?, target?, watch?, watchOptions? }
 - configuration[1] has an unknown property '0'. These properties are valid:
   object { amd?, bail?, cache?, context?, dependencies?, devServer?, devtool?, entry?, externals?, infrastructureLogging?, loader?, mode?, module?, name?, node?, optimization?, output?, parallelism?, performance?, plugins?, profile?, recordsInputPath?, recordsOutputPath?, recordsPath?, resolve?, resolveLoader?, serve?, stats?, target?, watch?, watchOptions? }

 Command failed with exit code 1: webpack-cli --config /Users/gozala/Projects/js-ipfs/node_modules/aegir/src/config/webpack.config.js --progress 

There might be other / better ways to go about it with aegir, but so far I'm struggling to see those.

[Gozala]

I would like to suggest adding --target webworker to aegir build (similar to aegir test --target webworker) that could produce build for webworker` target. Although that on it's own is probably not going to be enough. As we do need separate dist that spits out two files one for loading in the main thread and the other in the worker thread.