Only generate variants for non-user layers

[RobinMalfait]

This PR fixes a bug that generated variants for custom user css which is something we don't want. We only want this for css that's defined in a layer.

E.g.:

CSS:

@tailwind utilities;

.foo {
  color: red;
}

HTML:

<div class="hover:foo"></div>

This should not generate (but it did):

.hover\:foo:hover {
  color: red;
}

This PR will disallow that. If you do want this behaviour, then you would have to put the .foo class inside a layer. E.g.:
CSS:

@tailwind utilities;

@layer utilities {
  .foo {
    color: red;
  }
}

HTML:

<div class="hover:foo"></div>

This should generate:

.hover\:foo:hover {
  color: red;
}

[mnaderian]

@RobinMalfait, shouldn't it generate this? 🤔

.hover\:foo:hover {
  color: red;
}