[Bug]: `:where` is incorrectly purged when it is a child of `:not`

Describe the bug

When the pseudo-class :where is a child of :not and the selector inside of :where is an "unused" selector, it is incorrectly purged.

I would also assume that :is is also affected by this as both pseudo-classes are treated similarly.

To Reproduce

.root :not(:where(.unused)) {
    color: purple;
}

Expected Behavior

The rule should not be purged.

Environment

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Add any other context about the problem here

n/a

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