Function lower_figure_directive 

pub(crate) fn lower_figure_directive(
    document: &mut Document,
    root: NodeId,
    args: &[SetArg],
    span: &SourceSpan,
    inputs: &mut ExternalInputs<'_>,
    em_pt: f64,
    diagnostics: &mut Vec<Diagnostic>,
)

Lower #figure(image: ..., caption: ...) into a figure node.

The figure gets an image child and a caption paragraph child.