CLN: Cleanup plotting submodule

[sinhrks]

• Make plotting as subpackage
  • Split utility functions / non- MPLPlot classes to other files.
• Move tests under plotting/*
  • Add small unit tests to guarantee utility function APIs.
• Refactor MPLPlot class properties. Because MPLPlot now has lots of properties, split them to FigureProperties and AxesProperties classes and delegates the logic.

This should makes #12547, #7670, #8018 easier.

[sinhrks]

Hopefully the cleanup should fix issues like #12565.

[TomAugspurger]

I've been wanting to do a cleanup for a while too :)

I won't really have time to take the lead on this, but I can at least commit to reviewing and testing everything.