parameterless constraints draft

[smiklosovic]

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[bbotella]

Not for this PR, and don't know if worth doing, but here it goes yet another crazy idea... What about something like:

(K_NOT)? funcName=ident

With the K_NOT optional we can "negate" whatever the func does. This would allow us to reuse the NOT notation for all other constraints. A crazy example would be:

ALTER TABLE ks.tb ADD val3 text CHECK NOT JSON