Support defining windowing behavior in the WINDOW clause

PostgreSQL and SparkSQL support defining the windowing behavior in the WINDOW clause and then referenced in OVER.
This is very useful if the same windowing behavior is wanted for several functions.

For example:

````
SELECT sum(salary) OVER w, avg(salary) OVER w
FROM empsalary
WINDOW w AS (PARTITION BY depname ORDER BY salary DESC);

````

https://www.postgresql.org/docs/15/tutorial-window.html

https://www.postgresql.org/docs/15/sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS

Start by tracing how the parser handles WINDOW and OVER syntax, using the PostgreSQL examples in the issue as the grammar reference. Define the required parsing behavior for named window definitions and references, then add coverage proving both functions can reuse w and that the example parses.

Written by the indexing model from the issue text.