[Feature]: support stats.compilation.chunkGroups

[mxmason]

What problem does this feature solve?

Plugins that rely on knowledge of chunkGroups do not work correctly. webpack-bundle-tracker, which my team relies on to build a Django app with Webpack, is one such broken plugin. Here's the line of source code where webpack-bundle-tracker references chunkGroups.

What does the proposed API of configuration look like?

Similar to the API in Webpack, which makes plugins like this possible.

Edit: I apologize if this feature is a duplicate of #5641. I just found that one, but I'm not sure it's talking about the same thing I am. I'm also not sure I understand the suggested workaround to use stats, since the stats object appears to not have the data I need when I log it. Additionally, I need the ability to read these chunks while running a dev server as well as in prod.