This PR fixes an infinite loop when you use a @variant inside of a @custom-variant, where the @variant used is another @custom-variant.

The issue stems from the fact that a @custom-variant can use a @slot that we have to replace with the proper AST nodes. However in this setup, the AST nodes will include a @slot node as well, which causes us to replace the @slot again, and so on, causing an infinite loop.

@custom-variant a {
  @slot;
}

@custom-variant b {
  @variant a {
    @slot;
  }
}

The solution here is to replace the @slot nodes and then skip walking the nodes that were just inserted. This does mean that we end up with a @slot node in the final AST but that's not a real issue because that will get replaced later when handling the next @custom-variant.

Test plan

1. Existing tests still pass
2. Added a regression test to ensure that the infinite loop does not happen anymore
3. Added additional tests to ensure that the behavior is correct

Thanks @wongjn for your initial debugging help and providing a test case as well!

Fixes: #19618