Forbid implementing methods from UnitEnum and BackedEnum

[weirdan]

Also forbid implementing methods from UnitEnum and BackedEnum
https://3v4l.org/k5i42
https://psalm.dev/r/d0aece15a6

This is technically allowed since it's a UnitEnum and not a BackedEnum.

Originally posted by @AndrolGenhald in #7722 (comment)

[psalm-github-bot]

I found these snippets:

https://psalm.dev/r/d0aece15a6

<?php

enum E
{
    public static function cases(): array
    {
        return [];
    }
}

Psalm output (using commit 5d1fe88):

No issues!