v4: Allow `@apply` to be used with CSS mixins (#19427)

The CSS Custom Functions and Mixins spec [is using `@apply` with dashed
idents](https://drafts.csswg.org/css-mixins-1/#apply-rule) for native
mixin support. We shouldn't attempt to treat these as utilities and try
to compile them since they'll fail.

There one intentional limitation with regards to mixins and our use of
`@apply`: We do not allow users to mix utilities and mixins in the same
at-rule. In other words, all of the following `@apply` rules are
invalid:

```css
.foo {
  /* Invalid because the rules contain both mixins and utilities */
  @apply --my-mixin underline;
  @apply --my-mixin() underline;
  @apply underline --my-mixin;
  @apply underline --my-mixin();
}
```

Aside: Lightning CSS does not yet support this syntax so the results of
a production build won't produce the correct code but we'll at least
handle these correctly in Tailwind CSS itself.

Fixes #19422

---------

Co-authored-by: Robin Malfait <malfait.robin@gmail.com>