improve pseudo element handling

[romainmenke]

The previous change to all this logic left the order of simple selectors of the same type in tact. This worked for this case :

:is(:hover, :focus)::before {
  color: red;
}

But didn't work for some other cases.
With all the tools available to stylesheet authors to compose selectors (nesting, :is, custom selectors, ...) it is better to be more forgiving and have an explicit order for pseudo elements vs pseudo classes.

[Antonio-Laguna]

Thanks for tackling this! Looks more solid