Add aria-* variants

[reinink]

This PR adds a new aria-{state} variant which makes it easier to style elements based on the states of ARIA attributes.

For example, you can update the background color of an element based on whether the aria-checked state is true:

<div aria-checked="true" class="bg-gray-600 aria-checked:bg-blue-600">
  <!-- ... -->
</div>

Included variants

By default Tailwind provides the variants for the most common boolean ARIA attributes:

Variant	CSS
aria-checked:	[aria-checked="true"]
aria-disabled:	[aria-disabled="true"]
aria-expanded:	[aria-expanded="true"]
aria-hidden:	[aria-hidden="true"]
aria-pressed:	[aria-pressed="true"]
aria-readonly:	[aria-readonly="true"]
aria-required:	[aria-required="true"]
aria-selected:	[aria-selected="true"]

Using custom values

You can customize which ARIA variants are available by editing theme.aria or theme.extend.aria in your tailwind.config.js file:

module.exports = {
  theme: {
    extend: {
      aria: {
        'asc': 'sort="ascending"',
        'desc': 'sort="descending"',
      }
    }
  }
}

Then you can use any custom ARIA variants you've configured in your project:

<th
  aria-sort="ascending"
  class="
    aria-asc:bg-[url('/img/up-arrow.svg')]
    aria-desc:bg-[url('/img/down-arrow.svg')]
  "
>
  <!-- ... -->
</th>

Arbitrary values

For more complex ARIA attributes that take specific values (like aria-activedescendant), or for situations where it just doesn't feel worth it to add an alias to your theme, you can use the arbitrary value/square bracket notation to create custom ARIA variants on the fly:

<th
  aria-sort="ascending"
  class="
    aria-[sort=ascending]:bg-[url('/img/up-arrow.svg')]
    aria-[sort=descending]:bg-[url('/img/down-arrow.svg')]
  "
>
  <!-- ... -->
</th>

Group and peer variants

The ARIA variants are also available as group and peer modifiers.

<div>Message:</div>
<div role="textbox" contenteditable aria-required="true" class="peer"></div>
<div class="hidden peer-aria-required:block">*A message is required</div>

[seb-jean]

I love this PR @reinink