Function panel_fn 

pub fn panel_fn(
    child: impl IntoUiNode,
    wgt_fn: impl IntoVar<WidgetFn<PanelFnArgs>>,
) -> UiNode

P Widget function that converts PanelFnArgs to a widget.

This generates the panel that contains all markdown blocks, it is the child of the Markdown! widget.

Sets the PANEL_FN_VAR.