Drop support for pseudo-elements with `@extend`

At some point in the future, we'd like to move towards generating :is() selectors as the output for @extend (https://github.com/sass/sass/issues/2473). However, currently we allow pseudo-elements as both the extender and target of an extension, which means they would end up in the generated :is() selector—something that's not allowed by the spec.

To address this, we should deprecate and then eventually drop support for pseudo-elements as the extender or target for extensions.