Additional Lightswitch customizations

[proimage]

After seeing #17492 in the 5.8 changelog, a couple of ideas came to me.

1. Allow optional icons for Lightswitch state labels, and reflect the current state's icon in the toggle (replacing the now-default checkmark).
2. Allow color selection for the Lightswitch states (as color chips? icon color?), and reflect the current state's color in the toggle's background (instead of the hard-coded gray & green).

I should note that I often present Lightswitches to content authors as more of a "this or that" selector (eg. an Image Width lightswitch field with states labeled "Normal Width" & "Full Width"), rather than a simple "on/off" thing (eg. Allow Image Full Width? with states of "No" & "Yes"). I have no problem with Lightswitches still being booleans at the data level; I just prefer to present them in a way that reduces the mental load on content authors. It's like how clicking a button in a Cookie Consent box that says "Yes" or "No" means you have to be aware of the question those buttons are answering; whereas a button that directly states "Allow Necessary Cookies" / "Allow All Cookies" makes it more straightforward.

[mattbloomfield]

Possibly related: #17743