feat: allow unknown on the is operator

Also make the IS values strict at the Parser level since IS cannot be
parametrized.

CHANGELOG.md

@@ -17,6 +17,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - #1075, Allow filtering top-level resource based on embedded resources filters - @steve-chavez, @Iced-Sun
    + This is enabled by adding `!inner` to the embedded resource, e.g. `/projects?select=*,clients!inner(*)&clients.id=eq.12`
    + This behavior can be enabled by default with the `db-embed-default-join='inner'` config option, which saves the need for specifying `!inner` on every request. In this case, you can go back to the previous behavior per request by specifying `!left`  on the embedded resource, e.g `/projects?select=*,clients!left(*)&clients.id=eq.12`
+- #1988, Allow specifying `unknown` for the `is` operator - @steve-chavez
 
 ### Fixed
 
@@ -586,4 +587,4 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Fixed
 - Make filter position match docs, e.g. `?order=col.asc` rather
-  than `?order=asc.col`.
+  than `?order=asc.col`.