IS Operator

TODO: flesh out doc

For predicates on nullable fields, we support IS [NOT] NULL, IS [NOT] UNKNOWN operator, and also IS [NOT] true/false. NULL and UNKNOWN are synonyms in these clauses. For Boolean value fields, IS [NOT] true/false can be used to assume default values for unset fields during Boolean predicates. For instance, x IS NOT true is equivalent to x IS NULL OR x = FALSE (effectively treating unset fields as equivalent to false).