Added ability to add prefix/postfix elements for ranges.

[5cript]

Makes this possible:

render(
    body{reference = parent}(
        range(vec)
            .before(
                Nui::nil(),
                div{}("Prefix2")
            )
            .after(
                div{}("Postfix1"),
                div{}("Postfix2")
            ),
        [&vec](long long i, auto const& element) {
            return div{}(std::string{element});
        }
    )
);

Prefix elements cannot be rerendered with this implementation, except for when the complete range is rerendered.
This deficiency is accepted for now, because this fills a big feature gap.