Introduce &:: operator for pseudo-elements.

[nsaunders]

Description

The &: operator, previously overloaded to support both pseudo-elements and pseudo-classes, has been modified to support only pseudo-classes. Pseudo-elements now require the &:: operator instead.

Design considerations

The motivation for this design change is twofold:

1. According to MDN, "double colons should be used" for pseudo-elements to distinguish from pseudo-classes. In my interpretation, this syntax is considered helpful for understanding the target of a selector, so that the name of the pseudo-selector is not the only way to distinguish whether it is a pseudo-class or a pseudo-element.
2. The implementation of the &: operator is greatly simplified, e.g. several type classes removed, including some fancy-pants fundep stuff.

Future plans

This might help open the door for providing some "escape hatches" as requested in #28. I would much rather offer unsafePseudoClass and unsafePseudoElement functions, for example, than a single unsafePseudo function, because the type of pseudo-selector is important for determining whether the selector can continue to be extended with additional pseudo-classes etc.

References

• Pseudo-elements - CSS | Cascading Style Sheets [MDN]

Code change checklist

• Any new or updated functionality includes corresponding unit test coverage.
• I have verified code formatting, run the unit tests, and checked for any changes in the examples.
• I have added an entry to the Unreleased section of the CHANGELOG.