Enable debug source code in editor

[domoritz]

From https://github.com/vega/vega-editor/issues/190

It can currently be quite difficult to debug Vega and Vega-Lite because the sources are minified. Even with sourcemaps, it is quite tedious.

Ideally, we minifiy all dependencies except for Vega and Vega-Lite.

[domoritz]

Another neat feature is that this would speed up compilation for Vega-Lite developers.

[algomaster99]

@domoritz What's the issue exactly?

[domoritz]

I already split up the dependencies but minification sometimes makes debugging quite annoying. I don't know exactly what the best solution is. I think someone with experience in debugging Vega or Vega-Lite needs to look at this.