Split analyzer from the treemap UI code

This issue is a place to discuss and track a possible separation of webpack analyzer plugin code from the reporter UI code.

This idea was triggered by a proposal to add sunburst charts to the UI, where @th0r and me eventually thought it might be interesting to split these two concerns to distinct packages:

Or that the UI that is opened could be configurable, so that I could build a totally separate npm package that would only contain the UI parts that could be plugged into this packages webpack parts.

That's a very interesting idea and, at first glance, it won't be too hard to split analyzer from report UI.

We can move all current UI code from webpack-bundle-analyzer into, say, webpack-bundle-analyzer-treemap-reporter.
webpack-bundle-analyzer will be responsible only for analyzing bundle and feeding this info to reporter-plugins whose responsibility will be showing it in any way.

And the nice thing is reporter is not limited to some UI stuff - it can do anything it want with provided data!

Start by mapping the existing webpack analyzer plugin code against the reporter UI code; the issue names no files, tests, or entry points. Define the package boundary and reporter data interface first, then confirm done means the analyzer and treemap reporter are separate packages without losing current functionality.