Move opacity modifier support into plugin theme() function

[philipp-spiess]

This PR moves support for opacity modifies from the CSS theme() function into the plugin theme() implementation, this will allow plugins to use this, too:

let plugin = plugin(function ({ addUtilities, theme }) {
    addUtilities({
      '.percentage': {
        color: theme('colors.red.500 / 50%'),
      },
      '.fraction': {
        color: theme('colors.red.500 / 0.5'),
      },
      '.variable': {
        color: theme('colors.red.500 / var(--opacity)'),
      },
    })
  })
}

There's a small behavioral change for the CSS theme() function. Since tuples are resolved by default for the CSS theme() function only, these will no longer have opacity applied to their first values. This is probably fine given the reduced complexity as I don't expect the first values of tuples to be colors and the fix would mean we would have to parse the modifier in different places.