sqlsmith: test EOWC

[kwannoel]

Start with simple queries like select *, lead(x) over (partition by foo order by bar) from t.

Complex queries may be unsupported. e.g.

create mv emit on window close as
select max(s) from (
  select *, lead(x) over (partition by foo order by bar)
  from t
) group by bar;

[xxchan]

BTW, we can them link the doc instead of #7934 in the PR template later. 🤣

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