Compatible defaults for LEAD/LAG

I noticed that the PostgreSQL entry in a pan-database feature matrix by
Modern SQL was not reflecting the reality of our features.[1]

It turns out that test case used by the author produced an error which
the tool took to mean the feature was not implemented. I don't have the
actual test, but here is a simulation of it:

postgres=# SELECT LAG(n, 1, -99) OVER (ORDER BY n)
postgres-# FROM (VALUES (1.1), (2.2), (3.3)) AS v (n)
postgres-# ORDER BY n;

ERROR: function lag(numeric, integer, integer) does not exist
LINE 1: SELECT LAG(n, 1, -99) OVER (ORDER BY n)
^
HINT: No function matches the given name and argument types. You
might need to add explicit type casts.

Attached is a patch that fixes this issue using the new anycompatible
pseudotype. I am hoping this can be slipped into 13 even though it
requires a catversion bump after BETA1.

I looked for other functions with a similar issue but didn't find any.

[1] https://twitter.com/pg_xocolatl/status/1266694496194093057
--
Vik Fearing