Add per-side border classes

[ben-rogerson]

This PR adds the ability to specify border color and border width on a specific border direction:

import tw from "twin.macro";

// Colors
tw`border-t-gray-50`;
tw`border-b-red-500`;

// ↓ ↓ ↓ ↓ ↓ ↓

({
  "--tw-border-opacity": "1",
  "borderTopColor": "rgba(249, 250, 251, var(--tw-border-opacity))"
});
({
  "--tw-border-opacity": "1",
  "borderBottomColor": "rgba(239, 68, 68, var(--tw-border-opacity))"
});

import tw from "twin.macro";

// Widths
tw`border-t-4`;
tw`border-b-4`;

// ↓ ↓ ↓ ↓ ↓ ↓

({
  "borderTopWidth": "4px"
});
({
  "borderBottomWidth": "4px"
});