Function page_fn 

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

P Widget function that converts PageFnArgs to widgets.

A page is a stack of a maximum of lines_per_page, the text is split in pages mostly for performance reasons.

Sets the PAGE_FN_VAR.