Support theme values for has variant

[crswll]

This allows you to add values for the has variant to your theme so we can do things like:

theme: {
  has: {
    headers: ':where(h1, h2, h3, h4, h5, h6)'
  }
}

<div class="has-headers:bg-green-500">...</div>

over using arbitrary values all the time like:

<div class="has-[:where(h1,h2,h3,h4,h5,h6)]:bg-green-500">...</div>