Add new ring classes

[ben-rogerson]

This PR adds the entire set of new ring classes:

• Ring Width
• Ring Color
• Ring Opacity
• Ring Offset Width
• Ring Offset Color

How it works

To make these ring classes work, a bunch of css variables are added to every element using the <GlobalStyles /> import:

* {
  --tw-ring-inset: var(--tw-empty,/*!*/ /*!*/);
  --tw-ring-offset-width: 0px;
  --tw-ring-offset-color: #fff;
  --tw-ring-color: rgba(59, 130, 246, 0.5);
  --tw-ring-offset-shadow: 0 0 #0000;
  --tw-ring-shadow: 0 0 #0000;
}

Then when you use one of the new classes like ring, it will use the default css variables until you specify an overriding class:

tw`ring ring-offset-4`

// ↓ ↓ ↓ ↓ ↓ ↓

{
  "--tw-ring-offset-shadow": "var(--tw-ring-inset) 0 0 0 var(--tw-ring-offset-width) var(--tw-ring-offset-color)",
  "--tw-ring-shadow": "var(--tw-ring-inset) 0 0 0 calc(3px + var(--tw-ring-offset-width)) var(--tw-ring-color)",
  "boxShadow": "var(--tw-ring-offset-shadow), var(--tw-ring-shadow), var(--tw-shadow, 0 0 #0000)",
  "--tw-ring-offset-width": "4px"
}