Function find_window_nodes_within_select 

pub(crate) fn find_window_nodes_within_select<'a>(
    plan: &'a LogicalPlan,
    prev_windows: Option<Vec<&'a Window>>,
    already_projected: bool,
) -> Option<Vec<&'a Window>>

Recursively searches children of LogicalPlan to find Window nodes if exist prior to encountering a Join, TableScan, or a nested subquery (derived table factor). If Window node is not found prior to this or at all before reaching the end of the tree, None is returned.