Distinguishing pseudo-element vs pseudo-class in CSS selectors

[bershanskiy]

In #3496 @Elchi3 and @jpmedley proposed classifying types of API features to automatically generate article slugs and compat table element descriptions. I believe it would be nice to also distinguish pseudo-element and pseudo-selectors among CSS selectors. Specifically, this would enable us to automatically generate descriptions and article like "<code>:selector</code>" or "<code>::selector</code>" and slugs like :selector and ::selector.

[Elchi3]

Closing this as a duplicate of #3496