Change fs.watch to node-watch plugin

node doesn't have recursive support with linux

Refactor loader to a full webpack plugin. Declares aliases for kotlin module names and watches sources to re-run gradle build.

Add SourceMaps extracting from zip

Make sure loader is installed even if webpack build is initiated from Gradle, especially after clean.

build setup will create wp loader file to be configured in webpack.config.js. Doesn't seem to be necessary to deploy to npm.