Add Legacy Usage Example

Nearly identical to https://github.com/intuit/postcss-themed/issues/22 - you could do both!

One area where this project struggles with documentation is useful examples. We document bits and pieces of the PostCSS configuration required to use this project, but it doesn't really give a full picture for somebody new to PostCSS in general.

I would like to create a folder in the repository called `examples` and in it add a folder called `legacy` which has a simple app using the modern theme configuration and PostCSS.

This would require:

- Creating the folders.
- Create a sample application using Create React App, Webpack, or similar.
- Set up PostCSS using this plugin (with comments in the PostCSS config and code).
- Set up the browserlist to use the legacy theme generation.
- Create a simple page that contains a button that has themes for different products (maybe TT and QB), and a light and dark mode.
- Toggles to switch between themes and color schemes to show it working.

I think this would greatly simplify setup for people using this project or PostCSS for the first time if they need IE11 support.