Consider using named args in API when adding variables.

[chriscox]

e.g.

remixer.addRangeVariable({
  label: "Box opacity",
  defaultValue: 1,
  from: 0,
  to: 1,
  incrementSize: .1,
  onChange(nextValue) {
    box.style.opacity = nextValue;
  }
});

from https://github.com/material-foundation/material-remixer-web/pull/25#discussion_r89908054