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

[tkrotoff]

Fix #23371, 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 {
  ...
}

[tkrotoff]

A better PR has been already proposed and closed: #26471
Why was it rejected considering:

• "If there’s a way we can support more, I’m happy to hear it"
• "move to mixin so we can repeat it easier and allow folks to extend to custom states"

[mdo]

Thanks, but still inclined to leave this out of v4. I still don't see enough value in warning users of something that's not invalid and not valid. Other states just seem like they'd add confusion. Thanks again though!

[tkrotoff]

Other states just seem like they'd add confusion

It's not about adding other states to Bootstrap itself. It's about removing a limitation for the ecosystem.