Add slash alpha

[ben-rogerson]

This PR adds the slash alpha syntax which gives you a super quick way to set the alpha on colors.

import tw from "twin.macro";

// Use an opacity from your tailwind config by including it after the forward slash
tw`text-red-500/25`

// Use a custom alpha value by surrounding it with square brackets
tw`text-red-500/[.555]`

// Use a css variable using var within the square brackets
tw`text-red-500/[var(--myvar)]`

// ↓ ↓ ↓ ↓ ↓ ↓

({
  "color": "rgba(239, 68, 68, 0.25)"
});

({
  "color": "rgba(239, 68, 68, .555)"
});

({
  "color": "rgba(239, 68, 68, var(--myvar))"
});

The slash alpha feature works with these types of classes:

• tw`placeholder-red-500/25`
• tw`bg-red-500/25`
• tw`from-red-500/25`
• tw`via-red-500/25`
• tw`to-red-500/25`
• tw`border-red-500/25`
• tw`ring-red-500/25`
• tw`ring-offset-red-500/25`
• tw`text-red-500/25`
• tw`stroke-red-500/25` (with tailwind.config adjustment: stroke: theme => theme('colors'),
• tw`fill-red-500/25` (with tailwind.config adjustment: fill: theme => theme('colors'),