Add aria-busy variant

This is useful to show a spinner or dim a region that is updating.

Very useful in conjunction with [Turbo](https://turbo.hotwired.dev/reference/attributes#automatically-added-attributes)

Author: rik

stubs/config.full.js

@@ -15,6 +15,7 @@ module.exports = {
       bounce: 'bounce 1s infinite',
     },
     aria: {
+      busy: 'busy="true"',
       checked: 'checked="true"',
       disabled: 'disabled="true"',
       expanded: 'expanded="true"',