fix(issue#4354): unknown at-rule expression commas

[puckowski]

What:

Fix issue #4354 unknown at-rule expressions should not have commas in keyword list.

Why:

Current behavior (4.4.0+):

.box {
  @apply h-64, w-64;
}

Expected behavior:

.box {
  @apply h-64 w-64;
}

There have been downstream issues, see: https://github.com/web-infra-dev/rsbuild/issues/5688

Checklist:

• [ ] Documentation
• [x] Added/updated unit tests
• [x] Code complete

Should be a less hacky solution than https://github.com/less/less.js/pull/4366 Seems some logic was not needed as tests passed without some problematic code. I added some additional layer at-rule tests since the regression was caused by code ntroduced to support layer at-rules.

[puckowski]

@matthew-dean I know we have a major test refactor pending but I hope you can review the general idea of this PR in your spare time since this bug is impactful.

[puckowski]

Closing in favor of https://github.com/less/less.js/pull/4389