Add enabled variant

[adamwathan]

This PR adds a new enabled variant for the :enabled pseudo-class.

Originally we decided to skip this because it felt simpler to just let people make their default styles the enabled styles and explicitly style disabled styles using the disabled variant, but there are situations where it is more terse to use enabled, specifically when you want to target another pseudo-class but not when the element is disabled:

<!-- Before -->
<button class="bg-blue-500 active:bg-blue-600 disabled:active:bg-blue-500">

<!-- After -->
<button class="bg-blue-500 enabled:active:bg-blue-600">

Resolves #7903.

[holic]

🙏 I've wanted this for so long!