feat(color-extension): add class names

Created by: barillax

We have a custom extension that sometimes sets the color of containing elements to green (in addition to some other interactive behavior it adds). However, we wanted to be able to defer to overridden colors that were set by the color.js extension. In pseudocode:

$ x + \mycustomextension[y] $

y is green as a side effect of \mycustomextension

$ x + \color{red} \mycustomextension[y] $

y is red due to \color

To accomplish this, we needed color.js to set a class, mjx-extension-color on the .mjx-mstyle element that it creates so that we could adjust our CSS to color: inherit when .mjx-extension-color is an ancestor. Without this class, we can't distinguish between overridden colors and overridden sizes, for example.

This PR includes the simple changes to color.js to accommodate this. I'm open to suggestions for a better class name than mjx-extension-color.