Support window definitions with aliases

[alamb]

Originally posted by @jonahgao in #15033 (comment):

Unrelated to the current fix, we should compare them using normalized names to support the following SQL

SELECT
  t1.v1,
  SUM(t1.v1) OVER W + 1
FROM
  generate_series(1, 5) AS t1(v1)
WINDOW
  w AS (ORDER BY t1.v1);

[chenkovsky]

take