Function modifier_fn 

pub fn modifier_fn(
    child: impl IntoUiNode,
    modifier_fn: impl IntoVar<WidgetFn<ModifierFnArgs>>,
) -> UiNode

P Widget function that converts a ModifierFnArgs to a widget.

This is used for both the Shortcut::Modifier standalone and the KeyGesture::modifiers flags.

This property sets the MODIFIER_FN_VAR.