[STUDIO-4142] Change the visual appearance of toggle groups

[tristen]

Before	After

Summary of changes

• Sizes now match inputs:
  • toggle--s is 18px tall
  • toggle is 24px, matching input--s (down from 30px)
  • toggle--l is new and 36px tall, matching input
• Content is centered, so icon labels line up without extra classes.
• Light variants work on dark backgrounds: toggle--lighten* and toggle--white get a translucent track,
  dark text on the checked segment, and lighten25 disabled text. toggle--darken* gets a darken5 track.
• toggle--active-{color}`` selectors now include .toggle`, so they still override the light-variant text
  color. This only affects markup that uses the class without .toggle.