Function emit_reference_image 

pub fn emit_reference_image(
    builder: &mut impl InlineSink,
    alt_text: &str,
    label: &str,
    gap: &str,
    is_shortcut: bool,
    config: &ParserOptions,
    suppress_footnote_refs: bool,
)

Emit a reference image node with registry lookup. gap carries whitespace consumed between ] and [ under spaced_reference_links; empty otherwise.