selectr - Translate CSS Selectors to XPath Expressions

Translates a CSS selector into an equivalent XPath expression. This allows us to use CSS selectors when working with the 'XML' and 'xml2' packages, which can only evaluate XPath expressions. Also provided are convenience functions for querying XML and HTML documents with CSS selectors. This package was originally a port of the Python package 'cssselect' (<https://cssselect.readthedocs.io/>).