Support `IS NOT TRUE`, `IS NOT FALSE`, and `IS [NOT] UNKNOWN`

Hi, I would like to be able to parse something like:
```
SELECT
    b IS TRUE AS t,
    b IS FALSE AS f,
    b IS NOT TRUE AS nt,
    b IS NOT FALSE AS nf,
    b IS UNKNOWN AS u,
    b IS NOT UNKNOWN AS nu
FROM data
```
But we are unable to parse `IS NOT TRUE`, `IS NOT FALSE`, and `IS [NOT] UNKNOWN`.