Update content parsing

• Changes element(identname) to element-ident(identname) so as not to conflict with the parsing for https://developer.mozilla.org/en-US/docs/Web/CSS/element
• Switches to using generate css variables for target-counter and target-text to allow chaining them with strings, such as "(See p. " target-counter(attr(href), page) ")"
• Escapes newlines and quotes in target-text