Add support for named window frames

[berkaysynnada]

Parser cannot build a statement for named window frames. The query below can run successfully,

SELECT SUM(amount) OVER(ORDER BY ts) as sum1 FROM sales_us;

However, the equivalent query below with a named window frame cannot be run.

SELECT SUM(amount) OVER running_window FROM sales_us WINDOW running_window AS (ORDER BY ts);

Postgres supports both versions.