Add `rel.True`, `rel.False`, `rel.NotTrue`, `rel.NotFalse` filters

In PG correct way to generate boolean comparison via predicate`IS`, like `IS TRUE`, `IS NOT TRUE`, etc.

https://www.postgresql.org/docs/9.0/datatype-boolean.html

Start by locating the existing rel predicate or filter implementations that define comparable helpers, then read the PostgreSQL boolean documentation linked in the issue. Add the four requested filters so they represent IS TRUE, IS FALSE, IS NOT TRUE, and IS NOT FALSE, and verify their generated predicates with the repository's existing tests.

Written by the indexing model from the issue text.