Add is not? bool comparison functions

[bvolpato]

Having the functions is true / is false / is not true / is not false would be really useful.

They are somewhat common functions: https://www.postgresql.org/docs/current/functions-comparison.html:

And we're having some problems using isthmus without them, because Calcite has some rules that explicitly convert CASE statements to IS TRUE (e.g., AggregateCaseToFilterRule), and we need to solve through extensions.

I plan to take a stab at this, but let me know if you have any comments / ideas.

[westonpace]

I agree these will be useful to have.