Function try_parse_reference_image 

pub fn try_parse_reference_image(
    text: &str,
    allow_shortcut: bool,
    allow_spaced: bool,
) -> Option<(usize, &str, String, &str, bool)>

Try to parse a reference-style image: ![alt][ref], ![alt][], or ![alt] Returns (total_len, alt_text, label, gap, is_shortcut) if successful. gap is the whitespace between ] and [ consumed under spaced_reference_links; empty otherwise (and always empty for shortcuts).