Fix parsing nested forgiving grammars

`:is(:not(:is(:unknown()), valid))` was parsed as `:is()` containing an invalid
selector (but serializing as is), because the parse error resulting from
`:unknown()` was propagated above `:not()`, since the top-level `:is()` is
forgiving. This was a wrong optimization.

Author: cdoublev

lib/parse/parser.js

@@ -1106,7 +1106,7 @@ function parseCSSValue(input, definition, context, strategy = 'backtrack') {
     switch (root.state) {
         // The top-level list is guaranteed to be invalid (match is SyntaxError)
         case 'aborted':
-            return (forgiving && !context.function?.context.forgiving) ? null : match
+            return forgiving ? null : match
         // The current list is invalid (match is null or undefined)
         case 'rejected':
             return type === 'declaration' ? parseCSSValueSubstitution(input, context) : null