Fix pseudo-element specificity

[parzh]

See microsoft/vscode#72608 (comment) for a quick reference

In CSS3, pseudo-classes (such as :hover, :focus, or :invalid) and pseudo-elements (such as ::before, ::after etc.) have some noticeable differences:

• pseudo-classes are more specific than pseudo-elements;
• pseudo-classes are prefixed with :, while pseudo-elements start with :: (though for backwards compatibility this is not enforced);

Currently, they are being distinguished by amount of semicolons, which produces incorrect results when encountering backwards-compatible code:

Specificity here is expected to be (0, 0, 3), but got (0, 1, 2). That's because pseudo-element was incorrectly parsed as pseudo-class.

Here the number of colons (::) corresponds to the entity type (pseudo-element), so the specificity is displayed correctly.

This PR fixes this issue.

[msftclas]

All CLA requirements met.

[aeschli]

Thanks @parzh . I modified the RP a bit in order to keep the changes minimal. I hope that's ok for you.

[parzh]

@aeschli Thanks for the merge! Yes, that's fine.