Add "absent" and "present" field operators to the Ferret query compiler

Summary: Ref T13509. Parse "xyz:-" as "xyz is absent" and "xyz:~" as "xyz is present". These are new operators which the compiler emits separately from "not" and "substring".

Test Plan: Added unit tests, ran unit tests.

Maniphest Tasks: T13509

Differential Revision: https://secure.phabricator.com/D21107