Support PREV/NEXT in PyDough

[knassre-bodo]

Issue #157 is a pre-requisite for this issue since it starts the handling of some of the window aspects and has overlapping logic. The idea is that PREV accesses a singular collection corresponding to the previous record of the current collection, and can be used in a similar manner as BACK. By default it takes in no arguments, but can have the following:

• Positional argument n: for PREV, this is how many records to go back (e.g. PREV(n).x is equivalent to LAG(x, n)). The default is 1. NEXT(n) is equivalent to PREV(-n).
• by: see RANKING (order keys)
• level: see RANKING (partition keys)

The implementation will have components in the unqualified node stages, qualification, QDAG, hybrid nodes, hybrid conversion, relational nodes, and relational conversion, but ideally much of what was done for RANKING can be re-used. For this issue, anything other than references to expressions from prev/next can be forbidden (e.g. calling HAS or accessing subcollections can be left for a followup issue, which should be opened if they are not handled in the original implementation).

Example usages:

# Finds the average time difference between all orders of parts meeting certain criteria
selected_lines = lineitems.where(CONTAINS(part.name, 'green') & CONTAINS(part.part_type, 'STEEL'))
time_differences = selected_lines.ORDER_BY(ship_date.ASC())(delta=DATEDIFF('day', ship_date, PREV(1).ship_date))
TPCH(avg_delta=AVG(time_differences.delta))

# Finds the average time difference between all orders made per-customer
time_differences = orders(delta=DATEDIFF('day', order_date, PREV(1, by=order_date.ASC(), levels=1).order_date))
customers(name, avg_order_gap=AVG(time_differences.delta))