fix(issue#4354): unkown 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: web-infra-dev/rsbuild#5688

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

Kept signature of AtRule largely the same to not introduce regressions.

[Coiggahou2002]

This fix is ​​very important for less. If it is not fixed, less will become incompatible with Tailwind's custom at-rules (such as @apply which relies on splitting the keywords after @apply by space/tab/linkbreak).

[matthew-dean]

It feels a little like a hack (as in-, why is Less assuming a list in the first place? That we then need to "override"?) But it's probably ok for now. (@puckowski can you add in some JSDoc stuff about how this is an odd behavior to begin with?)