Warning when :is() contains pseudo-elements

[yisibl]

input

.foo:is(::before) {
  color: green;
}

output

.foo:is() {
  color: green;
}

expected

Prints a warning:

Pseudo-elements are not valid within the :is() selector.

.foo:is(::before) {
  color: green;
}

This is also not valid in CSS nesting, so you should print a warning when the same situation is encountered when nesting.

This is a clear departure from the traditional CSS preprocessor(SCSS/Less), and as CSS nesting becomes more widely implemented in browsers, more and more authors will be confused(see #352), even developers of browser engines: https://issues.chromium.org/issues/40278599

So we should explicitly tell them that they can't write this way in browsers at the moment.