unusedSymbols should also remove unused selectors when combined with a used selector

Hi, I've been trying to reduce the size of my minified CSS bundle via the unusedSymbols option, and I've noticed that lightningcss won't remove classes in the unusedSymbols list, if they are combined with a selector that is used.

If I set unusedSymbols to ["foo"] And then run lightningcss on this css file:

.foo, .bar {
  color: red
}

I would assume the output would be:

.bar {
  color: red
}

Instead, the output still contains the .foo class:

.foo, .bar {
  color: red
}

here's a repro using the playgound

Start by reproducing the unusedSymbols case with a comma-separated selector containing one unused and one used class. Add a regression test showing that the unused selector is removed while the used selector and declaration remain, then verify the existing test suite passes.