Fix form-validation-state() for unknown pseudo classes

Created by: tkrotoff

Fix #23371 (closed), see also Stack Overflow - Bootstrap 4, is-pending form validation state

Make things like @include form-validation-state("warning", ...) possible: $state can now be anything not just valid and invalid.

It changes nothing for valid and invalid: the CSS output is exactly the same.

Output for @include form-validation-state("warning", ...):

• before (:warning is an unknown pseudo class => the browser will ignore the whole line)

.was-validated .form-control:warning, .form-control.is-warning {
  ...
}

• after (:warning replaced by .is-warning => no impact)

.was-validated .form-control.is-warning, .form-control.is-warning {
  ...
}