Color output: Provide a way to access the computed color

[tylergaw]

This tool is about using color-mix so the focus is on showing the CSS to produce that code. However, it would be helpful to be able to see the computed color of the color-mix function. We'll be able to get at this either using colorjs or by reading the computed background color of the output element.

TBD what the exact UI will be here, but since it's a secondary action, it will likely be behind a click. Doesn't feel like it needs to be surfaced by default.