Re: Add RESPECT/IGNORE NULLS and FROM FIRST/LAST options

The last time this was discussed (https://www.postgresql.org/message-id/1037735.1610402426%40sss.pgh.pa.us) it was suggested to make the feature generalizable, beyond what the standard says it should be limited to.

With it generalizable, there would need to be extra checks for custom functions, such as if they allow multiple column arguments (which I'll add in v2 of the patch if the design's accepted).

So I think we need a consensus on whether to stick to limiting it to several specific functions, or making it generalized yet agreeing the rules to limit it (such as no agg functions, and no functions with multiple column arguments).