Add documentation and flags on how to debug client

Problem/Motivation

Debugging the client in interaction with Drupal can be hard. It would be nice to be able to easily build a dev-build that can be embedded

Proposed resolution

Add flags/env variables to build a dev-build with inline source maps so that debugging is easier.

Remaining tasks

Please find a patch attached. Documentation should be to document how to build a dev build.

User interface changes

Building on the GENERATE_SOURCEMAP-env variable (which tho' imho doesn't work) the patch adds the MINIFY, COMPACT, SOURCE_MAP env variables so you can adjust the build process as follows:

By running `GENERATE_SOURCEMAP=1 MINIFY=0 COMPACT=0 SOURCE_MAP=inline-source-map npm run build` you then get an easily debuggable (if also huge) build artifact that can be linked/copied to Drupal code based.

API changes

n/a

Data model changes

n/a