swap values

code.js

//..
let foregroundAlpha
let backgroundAlpha
//...
  if (msg.type === 'swap') {
    if (figma.currentPage.selection.length > 1) {
      ;[foregroundColor, backgoundColor, foregroundAlpha, backgroundAlpha] = [
        backgoundColor,
        foregroundColor,
        backgroundAlpha,
        foregroundAlpha,
      ]
      const contrast = calculateContrast(foregroundColor, foregroundAlpha, backgoundColor)
      sendContrastInfo(contrast, foregroundColor, backgoundColor)
    }
  }
}