Color closures

[ben-rogerson]

This PR adds support for custom colors that work with tailwind opacity classes added in Tailwind v1.7.0.
You'll now be able to add custom colors to the tailwind config and then style their opacity with classes like text-opacity-50 / bg-opacity-50.

// tailwind.config.js
module.exports = {
  theme: {
    colors: {
      custom: ({ opacityVariable }) =>
        `rgba(255, 255, 255, var(${opacityVariable}, 1))`,
    },
  },
}

// usage
tw`text-custom text-opacity-50`

// output
({
  "color": "rgba(255, 255, 255, var(--text-opacity, 1))",
  "--text-opacity": "0.5"
});