fix: Improve devServer config validation (#504)
fix: Improve devServer config validation (#504)

- Allowed `devServer.hotEntries` to be a list of varying length
- Added `historyApiFallback` to enable custom routing for single-page apps
- Changed `open` to allow strings, as is allowed by `webpack-dev-server`
egoist/poi,egoist/poi,egoist/vbuild,egoist/vbuild,egoist/poi,egoist/poi,egoist/vbuild